UNIVERSITY  OF   CALirOBMIA  PPBLICATI0N8 

COLLEGE  OF  AGRICULTURE 

AGRICULTURAL  EXPERIMENT  STATION 

BERKELEY,  CALIFORNIA 


GUM  DISEASES  OF  CITRUS  TREES 

IN  CALIFORNIA 


BY 

HOWARD    S.    FAWCETT 


BULLETIN  No.  360 

April,  1923 


UNIVERSITY  OF  CALIFORNIA  PRESS 

BERKELEY,  CALIFORNIA 

1923 


David  P.  Barrows,  President  of  the  University. 

EXPERIMENT  STATION  STAFF 

HEADS  OF  DIVISIONS 

Thomas  Forsyth  Hunt,  Dean. 

Edward  J.  Wickson,  Horticulture  (Emeritus). 

,  Director  of  Resident  Instruction. 

C.  M.  Haring,  Veterinary  Science,  Director  of  Agricultural  Experiment  Station. 

B.  H.  Crocheron,  Director  of  Agricultural  Extension. 

C.  B.  Hutchison,  Plant  Breeding,  Director  of  the  Branch  of  the  College  of 

Agriculture  at  Davis. 

H.  J.  Webber,  Sub-tropical  Horticulture,  Director  of  Citrus  Experiment  Station. 
William  A.  Setchell,  Botany. 
Mybr  E.  Jaffa,  Nutrition. 
Ralph  E.  Smith,  Plant  Pathology. 
John  W.  Gilmore,  Agronomy. 
Charles  F.  Shaw,  Soil  Technology. 
John  W.  Gregg,  Landscape  Gardening  and  Floriculture. 
Frederic  T.  Bioletti,  Viticulture  and  Fruit  Products. 
Warren  T.  Clarke,  Agricultural  Extension. 
Ernest  B.  Babcock,  Genetics. 
Gordon  H.  True,  Animal  Husbandry. 
Walter  Mulford,  Forestry. 
James  T.  Barrett,  Plant  Pathology. 
W.  P.  Kelley,  Agricultural  Chemistry. 
H.  J.  QuAYLE,  Entomology 
Elwood  Mead,  Rural  Institutions. 
H.  S.  Reed,  Plant  Physiology. 
L.  D.  Batchelor,  Orchard  Management. 
W.  L.  Howard,  Pomology. 
*Frank  Adams,  Irrigation  Investigations. 

C.  L.  Roadhouse,  Dairy  Industry. 
R.  L.  Adams,  Farm  Management. 

W.  B.  Herms,  Entomology  and  Parasitology. 
John  E.  Dougherty,  Poultry  Husbandry. 

D.  R.  Hoagland,  Plant  Nutrition. 
G.  H.  Hart,  Veterinary  Science. 

L.  J.  Fletcher,  Agricultural  Engineering. 
Edwin  C.  Voorhies,  Assistant  to  the  Dean. 

DIVISION  OF  PLANT  PATHOLOGY 

J.  T,  Barrett  E.  T.  Bartholomew 

II.  S.  Fawcett  C.  O.  Smith 


*  In  cooperation  with  Division  of  Agricultural  Engineering,  Bureau  of  Public  Roads,  U.  S. 
Department  of  Agriculture. 


GUM  DISEASES  OF  CITRUS  TREES 
IN  CALIFORNIA* 


BY 
HOWAED  S.  FAWCETT 


CONTENTS  PAGE 

Introduction 370 

Pythiacystis  (brown  rot)  gummosis 371 

History 371 

Symptoms 373 

Investigations  into  the  nature  and  cause  of  the  disease 376 

Resistance  of  different  species  and  varieties 378 

Conditions  faciUtating  infection  and  development  of  the  disease 381 

Fusarium  as  a  secondary  aid  in  the  development  of  Pythiacystis  gummosis..  384 

Methods  of  control 385 

Prevention 385 

Treatment 387 

Mai  Di  Gomma  or  footrot 397 

Symptoms 397 

History 398 

Investigations 398 

Control 400 

Botrytis  gummosis 401 

Symptoms  and  occurrence 401 

Investigations  as  to  nature  and  cause 402 

Factors  favoring  the  disease 402 

Methods  of  control 404 

Prevention 404 

Treatm-nt 404 

Sclerotinia  gumming  due  to  Sderotinia  Ldbertiana 406 

Psorosis  (scaly  bark)  of  orange  trees 408 

Symptoms 408 

Investigations  as  to  cause  and  manner  of  development 408 

Experiments  in  treatment 410 

Suggestions  for  treatment 413 

Diplodia  gumming 416 

Twig  gumming 417 

Exanthema  or  dieback 418 

Nature  and  symptoms 418 

Control r. 419 

Minor  forms  of  gumming 419 

Gumming  due  to  Penicillium  roseum 419 

Gumming  due  to  Fusarium  sp 420 

Gumming  due  to  Alternaria  sp 420 

Gumming  due  to  Bacterium  citriputeale 420 

Gumming  associated  with  insect  injuries 421 

Gumming  associated  with  chemical  stimuli 421 

Physical  effects  of  the  environment 422 

Summary  of  directions  for  prevention  and  treatment 422 

*  Paper    Xo.    92,    University    of    California,    Graduate    School    of    Tropical 
Agriculture  and  Citrus  Experiment  Station,  Riverside,  California. 


370  UNIVERSITY    OF    CALIFORNIxV EXPERIMENT    STATION 


INTRODUCTION 

The  aim  of  this  bulletin  is  to  bring  together  the  information  now 
at  hand  regarding  the  so-called  gum  diseases  of  citrus  and  the  various 
minor  forms  of  gumming  in  citrus  trees  under  California  conditions. 
Special  emphasis  is  here  given  to  contributing  conditions  and  to 
methods  of  prevention  and  treatment.  A  considerable  amount  of 
investigational  data  not  heretofore  published  is  included.  The  results 
of  investigations  in  other  phases  of  these  diseases  are  being  published 
in  the  Journal  of  Agricultural  Research.  These  latter  phases  will 
therefore  receive  only  brief  treatment  here  in  order  to  avoid  undue 
duplication.  It  is  necessary,  however,  to  duplicate  in  part  the  descrip- 
tions of  the  diseases  and  a  small  amount  of  other  data. 

Previous  investigators  had  come  to  the  conclusion  that  all  gum 
diseases  of  citrus  trees  in  California  originated  independently  of 
micro-organisms.^  It  was  held  that  these  diseases  were  largely  auto- 
genous in  their  nature,  and  frequently  induced  through  the  effects  of 
certain  climatic  or  soil  conditions  alone.  It  now  appears  evident 
that  these  environmental  conditions  cannot  by  themselves  initiate 
the  severe  forms  of  gummosis  in  citrus  earlier  attributed  to  them, 
although  certain  factors  are  found  to  play  (as  they  do  in  most  para- 
sitic diseases)  an  important  role  as  contributing  conditions  which 
favor  infection  and  invasion  of  the  host  hy  the  causal  parasites. 

The  investigational  work  on  which  the  greater  part  of  this  bulletin 
is  based  was  begun  in  February,  1912,  under  the  direction  of  the  State 
Commission  of  Horticulture  and  continued  after  October,  1913,  at 
the  Citrus  Experiment  Station  of  the  University  of  California. 
Acknowledgment  of  the  assistance  rendered  during  this  investigation 
has  been  given  elsewhere.^ 

The  names  of  species  and  varieties  of  Citrus  will  be  used  in 
accordance  with  Swingle^  as  follows :  common  lemon.  Citrus  limonia 
Osbeck;  rough  lemon,  a  horticultural  variety  of  C.  limonia  Osbeck; 
sweet  orange,  C.  sinensis  Osbeck ;  sour  orange,  C.  grandis  Osbeck ; 
citron,  C.  medica  Linn.;  trifoliate  orange,  Poncirus  trifoHata  Raf. 
The  word  lemon,  when  used  alone  will  refer  to  the  common  lemon, 
and  the  word  orange  to  the  sweet  orange. 


1  Smith,  It.  E.,  and  Butler,  O.  Gum  disease  of  citrus  trees  in  California. 
Calif.  Agr.  Exp.  Sta.  Bull.  200,  pp.  235-272,  3908. 

2  Fawcett,  11.  S.,  Two  fungi  as  casual  agents  in  gummosis  of  lemon  trees  in 
California.  Monthly  Bull.  Calif.  State  Comm.  of  Horticulture,  vol.  2,  pp. 
601-617,  1913. 

3  Swingle,  W.  T.,  "Citrus,"  in  Bailey,  L.  H.,  Standard  Cyclopedia  of  Horti- 
culture, vol.  2,  pp.  270-785,  New  York,  1914. 


Bulletin  360]     gUM  DISEASES  OF  CITRUS  TREES  IN  CALIFORNIA  371 


PYTHIACYSTIS    (BROWN  ROT)    GUMMOSIS 

HISTOEY 

A  destructive  form  of  gum  disease  similar  to  Pythiacystis  gum- 
mosis  first  attracted  serious  attention  in  the  Azores  about  1834.  A 
similar  gum  disease  appeared  in  Italy  as  early  as  1863 ;  in  Portugal, 
1865 ;  in  Australia,  1867 ;  in  Spain,  1871 ;  in  the  United  States,  1865 ; 
and  in  most  other  citrus  regions  before  the  year  1890.  See  accounts 
and  references  by  Savastano^  Swingle  and  Webber,^  Butler^  and 
Fawcett.'^ 

In  the  early  history  of  citrus  growing  in  California  there  appears 
to  be  no  record  of  the  occurrence  of  gum  diseases  until  about  1875.^ 
Not  long  afterwards  they  became  an  important  hindrance  to  commer- 
cial citrus  culture,  as  is  evidenced  by  the  horticultural  literature  of 
the  time. 

A  committee  of  citrus  growers  appointed  to  examine  the  condition 
of  citrus  orchards,  stated  in  1878^  that  at  that  time  few  localities 
were  free  from  gum  disease,  which  this  committee  believed  was  caused 
by  excessive  irrigation  and  unsuitable  cultivation.  The  orchards 
most  heavily  flooded  with  irrigv.tion  water,  especially  on  heavy  soils, 
were  found  worst  affected.  The  application  of  manure  immediately 
around  the  trees  was  also  reported  as  tending  to  promote  the  disease. 
Lemon  roots  were  found  to  be  more  susceptible  than  orange  roots, 
and  in  a  discussion  which  followed  this  report  much  objection  is 
expressed  to  lemon  and  Chinese  lemon  stock  as  compared  with  sweet 
orange.  Light  soils  were  thought  by  many  to  be  better  adapted  than 
hea\'y  soils  for  orange  culture  since  orchards  on  light  soils  were 
healthy  while  those  on  heavy  soils  were  rapidly  dying  out,  presumably 
from  gum  disease. 

Irrigation  by  flooding  was  a  common  practice  at  that  time  and 
later^*^  it  was  stated  as  a  settled  fact  that  the  lemon  (on  lemon  roots) 
is  peculiarly  sensitive  to  moisture  and  easily  begins  to  rot  if  water  is 


4''Gommosi   degli   agrumi, "  in  Patologia   arborea   applicata,   bp.    127-141. 
Napoli,  1910. 

5  The  principal  diseases  of  citrus  fruits  in  Florida.     U.  S.  Dept.  Agr.  Div. 
Yeg.  Phys.  &  Path.  Bull.  8,  pp.  1-42,  1896. 

6  A  Study  on  Gummosis  of  Prunus  and  Citrus,  Ann.  Bot.,  vol.  25,  pp.  107-153, 
1911. 

7  Gummosis  of  Citrus,  Jour,  of  Agr.  Eesearch.     (In  press.) 

8  Mills,  J.  W.,  Citrus  fruit  culture.     Calif.  Agr.  Exp.  Station,  Bull.  13.8,  pp. 
1-46,  1902. 

9  Southern  Calif.  Horticulturist,  vol.  1,  p.  115,  Jan.,  1878. 

10  Southern  California  Horticulturist,  vol.  1,  pp.   314-315,  July,   1878. 


372  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

left  standing*  around  it.  In  the  following  year  the  statement  was 
made  in  the  same  publication^^  that  gum  disease  is  induced  by  mid- 
summer irrigation  and  that  lemon  trees  on  their  own  roots  were 
dying  rapidly.  In  1882  a  leading  nurseryman^^  regarded  gum  disease 
as  the  only  citrus  trouble  of  importance. 

Of  the  four  citrus  stocks  commonly  used  at  that  time  (sweet 
orange,  lemon,  lime  and  citron)  the  lemon  and  the  lime  were  stated 
by  Gary  to  be  the  most  fatally  affected  with  this  disease.  He  advises 
the  use  of  the  sweet-orange  stock  but  says  nothing  of  the  sour  orange 
which  was  introduced  at  a  later  date.  Gary's  description  of  the 
disease  and  his  reference  to  the  differences  in  susceptibility  of  varieties 
indicate  that  the  type  of  disease  spoken  of  was  Pythiacystis  gummosis. 
Because  of  the  susceptibility  of  the  lemon  the  growers  gradually  dis- 
covered that  other  stocks  must  be  used.  The  universal  verdict  accord- 
ing to  Holt^^  in  1892  was  that  lemons  should  not  be  grown  on  their 
own  roots. 

Sweet-orange  stock  though  much  less  susceptible  than  lemon  was 
also  frequently  affected  with  gummosis.  This  fact  drew  attention  to 
the  sour-orange  stock  which  had  been  used  successfully  in  southern 
Europe  and  in  Florida  to  replace  trees  affected  with  mal  di  gomma. 
This  resistant  stock  only  gradually  came  into  use  as  a  preventive  of 
gum  disease  in  California.  Cutter^^  in  1892  stated  that  his  attention 
w^as  called  first  to  the  value  of  the  sour-orange  as  superior  to  sweet 
in  its  resistance  to  gum  disease  in  1885.  The  following  year  the  firm 
of  Twogood,  Edwards  and  Cutter  of  Riverside,  received  the  first 
shipment  of  sour-orange  trees  from  Florida  for  commercial  purposes ; 
although  a  few  trees  had  been  grown  from  Florida  seeds  for  test  by 
others  before  this  time. 

In  the  light  of  our  present  knowledge  of  varietal  susceptibility  of 
citrus  to  gum  diseases  and  of  the  part  played  by  heaw  soils  and  by 
excessive  irrigation  (especially  flooding  on  heavy  soils),  the  failure 
in  growth  on  lemon  stocks,  and  the  dying  of  trees  on  heavy  soils, 
but  not  on  lighter  soils  Avould  now  appear  to  have  been  due  to  the 
presence  and  attack  of  gum  disease  organisms,  especially  Pythiacystis 
citropththora  Smith  and  Smith. 

The  early  discontinuance  of  the  use  of  lemon,  lime  and  citron  as 
stocks,   and  the  adoption   of  the  orange   as  a  general  stock  for   all 


11  Southern  Calif.  Hort.,  vol.  2,  pp.  83-86,  March,  1879. 

12  Gary,    Thomas    A.,    Orange    culture    in    California.      Pacific    Rural    Press, 
pp.   81-82,  San  Francisco,   1882. 

1-5  Holt,  L.  M.,  "Lemon  culture  for  profit,"  Proc.  Calif.  Pomological  Soc, 
1892. 

14  Cutter,  J.  E.,  Proc.  Calif.  Pom.  Soc,  1892. 


Bulletin  360]     gUM  DISEASES  OF  CITRUS  TREES  IN  CALIFORNIA  373 

varieties  appears  to  have  been  largely  brought  about  by  this  one 
disease.  Sour-orange  stocks  were  introduced  from  Florida  later,  as 
previously  indicated,  for  the  purpose  of  combating  this  disease  on 
heavy  soils,  but  this  stock  has  not  become  of  Avidespread  use  in  Cali- 
fornia until  recent  years. 

SYMPTOMS 

Pythiacystis  or  brown-rot  gummosis  with  its  associated  rot  of  the 
fruit^^  is  probably  the  most  widespread  and  destructive  of  the  citrus 
gum  diseases.  On  the  lemon  (the  most  susceptible  variety)  patches 
of  bark  on  the  trunk  are  kiued  and  often  large  quantities  of  gum  are 
exuded  (figs.  1  and  2a.)  Infection  usually  starts  at  the  base  of  the 
trunk  or  on  the  crown  roots  and  works  rapidly  both  upward  and 
laterally.  The  bark  is  killed  (not  as  in  cases  of  Psorosis  and  shell- 
bark,  merely  in  the  outer  cortical  layers)  but  entirely  through  to  the 
wood,  thus  including  the  cambium.  A  thin  layer  of  wood  tissue  only 
about  Vrz  of  an  inch  thick  is  visibly  affected  unless  secondary  organ- 
isms enter.  The  bark  above  the  soil  is  not  softened  as  it  is  in  the 
early  stages  of  Botrytis  gummosis  but  remains  firm  and  intact  until 
drying  causes  it  to  shrink  and  crack  longitudinally  (fig.  2h).  Below 
the  surface  of  the  soil  secondary  organisms  frequently  set  up  fermen- 
tation and  moist  decay.  On  the  bark  of  old  orange  trees  and  other 
partially  resistant  varieties  often  the  progress  of  the  disease  is  soon 
arrested  and  the  lesions  tend  to  become  self -limited.  The  loss  of  large 
patches  of  bark  is  followed  by  a  gradual  yellowing  and  dropping  of 
leaves  on  the  branches  leading  out  from  the  portion  of  the  trunk 
aifected. 

Although  these  are  the  main  characteristics  of  the  disease  as  seen 
on  superficial  examination,  there  are  many  special  features  which 
appear  when  more  careful  study  is  given  to  its  development  under 
various  conditions.  In  the  earlier  stages  of  the  disease  the  exuding 
gum  is  usually  the  only  external  symptom  (fig.  2a).  By  lightly 
scraping  the  bark  at  this  time  the  margin  between  the  sound  and 
invaded  tissue  is  shown  indefinitely,  only  by  the  gradual  shading  of 
the  normal  green  color  into  a  drab.  The  bark  is  not  softened  but 
remains  firm  and  only  after  a  considerable  time  does  it  shrink  and 
crack  longitudinally  (fig.  2h). 

On  healthy,  rapidly  growing  lemon  trees  the  area  of  killed  and 
darkened  bark,  which  is  elliptical  or  irregular  in  outline,  is  usually  5 
to  10  inches  in  vertical  length  and  half  that  in  width,  when  the  gum 

15  Smith,  R.  E.  and  others,  The  Brown  Eot  of  the  Lemon.  Calif.  Agr.  Exp. 
Sta.  Bull.  190,  70  pages,  1907. 


374  UNIVERSITY    OF    CALIFORNIA — EXPERIMENT   STATION 

first  becomes  apparent.  By  that  time  the  fungus  has  invaded  the 
the  bark  at  this  time  will  show  that  the  outer  margin  of  the  invaded 
zone  in  the  inner  tissues  is  about  coextensive  with  that  seen  on  the 
surface.  The  upward  and  downward  extension  from  the  point  of 
infection  is  usually  many  times  greater  than  the  lateral  extension. 

In  an  irregular  zone  or  band  surrounding  an  actively  invaded 
area,  the  cambium  layer  shows  an  influence  extending  from  the 
margins  of  the  dead  bark.  There  is  a  production  of  clear,  watery 
gum  which  seems  to  originate  in  the  region  of  the  embryonic  wood 
among  the  live  cells  without  any  apparent  fermentation  or  decay. 
This  region,  not  yet  darkened,  outside  the  invaded  portion,  will  be 
spoken  of  in  this  paper  as  the  ''outer  gummous  zone"  (fig.  3h).  It 
may  in  time  extend  considerable  distances  upward  and  downward  and 
small  distances  laterally  from  the  margin  of  the  invaded  zone  (fig. 
3&).  It  has  been  traced  for  2  and  3  feet  upward.  The  extent  of  this 
outer  gummous  zone  varies  with  the  age  and  rapidity  of  development 
of  the  disease  lesion,  the  condition  of  the  tree,  etc. 

The  inner  surface  of  the  bark  in  the  invaded  zone  in  a  lesion  of 
considerable  size  varies  in  color  from  mineral  brown  to  burnt  amber 
or  fawn^^  and  the  same  discolorations  will  be  found  on  the  surface  of 
the  wood  just  at  or  beneath  the  cambium  (fig.  Sh).  The  discoloration 
does  not  extend  far  (usually  only  %2  to  %  of  an  inch)  into  the  woody 
layers.  The  cambium  region  in  the  gummous  zone  is  chamois  to 
yellow  ochre  in  color,  gradually  fading  at  the  margins  into  the  normal 
color  of  the  sound  woody  surface. 

Frequently,  when  the  bark  is  irregular  in  contour,  gum  pockets 
1  to  2  inches  in  length  will  be  formed.  The  gum  accumulates  near 
the  cambium  and  by  pressure  separates  the  bark  from  the  wood  at 
certain  places,  forming  definite  pockets.  The  pressure  is  usually 
relieved  by  a  break  in  the  bark  before  the  pockets  become  large.  A 
few  deeper  gum  pockets  of  considerable  size  have  also  been  found, 
tissue  usually  for  a  period  of  from  2  to  4  months.  The  removal  of 
situated  in  the  outer  gummous  zone  beneath  layers  of  wood  %  to  i/4 
of  an  inch  in  thickness,  showing  accumulations  of  gum  under  pressure. 
The  gum,  which  is  watery  and  clear  when  first  formed,  hardens  as  it 
comes  to  the  surface,  apparently  through  loss  of  water,  and  finally 
becomes  brittle.  On  the  surface  the  hardened  gum  is  usually  mahog- 
any to  chestnut  in  color.^^  The  gum  accumulates  on  the  surface  in 
long  narrow  ridges  (figs.  1  and  2a)  or  in  oval  masses,  or  runs  down 
and  collects  in  masses  on  the  soil,  according  to  the  rapidity  of  its 


10  Ridgway,  Robert,  Color  standards  and  color  nomenclature,  43  pp.,  Wash- 
ington, D.  C,  1912. 


Bulletin  360]     qUM  DISEASES  OF  CITRUS  TREES  IN  CALIFORNIA 


375 


Fig.  1. — Pythiacystis  or  brown-rot  gummosis  on  lemon  trunk.  The  tree  is 
completely  girdled  the  dead  bark  resulting  from  several  infections  starting  near 
the  soil  surface.     Eidges  of  exuded  gum  are  characteristic. 


376  UNIVERSITY    OF    CALIFORNIA — EXPERIMENT   STATION 

formation  and  the  dryness  of  the  air.  During  periods  of  heavy 
dews  and  rains  it  gradually  dissolves  and  disappears.  The  invading 
hyphae  frequently  die  out  rapidly  behind  the  marginal  fringe  of 
advance,  and  quite  often  they  die  out  completely  over  a  part  or  all 
of  this  outer  margin,  so  that  the  progress  of  the  disease  is  checked  or 
entirely  arrested.  Such  cases  are  often  found  among  trees  having 
some  power  of  resistance,  especially  among  orange  and  pomelo  trees; 
or  where  the  weather  conditions  subsequent  to  infection  become 
unfavorable  to  the  parasite. 

In  trees  on  which  the  disease  has  been  present  for  a  long  time,  the 
dead  bark  over  the  invaded  portions  dries,  shrinks  and  cracks.  The 
larger  cracks  are  mostly  vertical,  with  smaller  horizontal  cracks 
(fig.  2&).  A  thin  layer  of  the  wood  immediately  under  the  invaded 
bark  will  usually  be  found  to  be  infiltrated  with  hardened  reddish- 
brown  gum  which  protects  the  under  layers  from  rapid  drying  and 
to  a  considerable  extent  against  the  entrance  of  wood  rotting  fungi. 

On  old  sweet-orange  trees,  the  invaded  areas  are  usually  less  exten- 
sive and  more  restricted  laterally  than  on  the  common  lemon.  There 
is  usually  less  gum  than  on  the  lemon.  In  the  orange  there  is  a  greater 
tendency  for  the  invading  fungus  to  die  out  and  for  the  invaded 
area  to  become  self -limited  than  in  the  lemon.  Frequently  the  invaded 
areas  on  old  sweet-orange  trunks  extend  upward  from  the  soil  surface 
as  narrow  tongues  of  killed  bark.  On  younger  orange  trees  and 
frequently  on  older  ones  growing  vigorously  on  heavy  clay  soils  the 
disease  may  assume  much  the  same  characteristics  as  it  does  on  the 
common  lemon. 

INVESTIGATIONS  INTO  THE  NATUEE  AND  CAUSE  OF  THE  DISEASE 

Investigations  begun  in  1912  by  the  author^^  have  led  to  the  dis- 
covery that  the  disease  is  infectious  and  that  the  casual  agent  is  a 
soil  inhabiting  'Svater  mold,"  Pythiacystis  citrophthora,  the  same 
fungus  which  was  previously  described  as  causing  brown  rot  of  lemon 
fruits.^^  The  relation  of  this  fungus  to  the  disease  was  discovered  as  a 
result  of  a  series  of  experiments  as  follows: 

1.  Inoculation  into  sound  trees  with  bits  of  diseased  tissue  trans- 
mitted the  disease  with  all  its  characteristic  symptoms  (fig.  2).  It 
was  found,  however,  that  only  the  diseased  tissue  from  the  marginal 


17  Fuwcett,  H.  S.,  Two  Fungi  as  Casual  Agents  in  Gummosis  of  Lemon  Trees 
in  California.  Month.  Bull.  Calif.  State  Comm.  of  Hort.,  vol.  2,  pp.  601-617, 
1913. 

18  Smith,  R.  E.  and  others.  The  Brown  Rot  of  the  Lemon,  Calif.  Agr.  Exp, 
Sta.  Bull.  190,  1917. 


Bulletin  360]     quM  DISEASES  OF  CITRUS  TREES  IN  CALIFORNIA 


377 


fringe  of  the  killed  bark  of  active  lesions  was  capable  of  transmitting 
the  disease.  Tissue  from  places  back  of  this  margin  toward  the 
center  of  large  lesions  or  from  the  outer  gummous  zone  was  incapable 


of  inducing  gummosis. 


Fig.  2. — Pylhiacystis  or  brown-rot  gummosis  on  lemon  tree.  Produced  by 
inoculation  with  diseased  bark  tissue. 

(a)  Tree  inoculated  February  27,  1912  and  photographed  April  24,  1912. 
The  large  amount  of  exuded  gum  indicates  the  rapid  development  of  a  gummosis 
lesion.  A  majority  of  the  trees  in  this  same  orchard  had  the  same  over  growths 
at  the  union  of  stock  and  scion  as  observed  in  this  illustration. 

(b)  Same  view  as  (a)  on  May  24,  1913,  15  months  after  inoculation.  The 
gum  first  formed  has  been  dissolved  by  winter  rains,  and  the  dead  bark  has 
dried  and  shrunken.     Gum  was  exuding  (farther  around)  at  this  time. 

2.  Culture  tests  made  from  pieces  of  bark  from  various  places 
within  and  beyond  the  killed  portion  of  the  lesions  revealed  the 
presence  of  Pythiacystis  citrophthora  but  usually  only  at  the  marginal 
fringe  of  what  was  subsequently  called  the  invaded  zone.  This  fungus 
usually  could  not  be  isolated  from  the  central  portion  of  the  invaded 
zone  and  it  was  never  found  well  out  of  the  outer  gummous  zone. 
Isolations  of  this  fungus  were  made  at  least  139  times  from  gummosis 
trees  representing  30  different  orchards  in  10  different  counties  of 
California  extending  from  San  Diego  on  the  south  to  Butte  county 
on  the  north.    It  was  isolated  also  from  one  locality  in  Arizona. 


378  UNIVERSITY    OF    CALIFORNIA — EXPERIMENT   STATION 

3.  Inoculations  were  made  into  bark  of  sound  trees  with  pure 
cultures  of  this  fungus  and  the  disease  was  induced  with  all  its 
characteristic  symptoms  (figs.  3  and  4a).  Many  repetitions  gave 
the  same  results.  Inoculations  with  large  numbers  of  other  fungi 
and  bacteria  found  in  the  older  portions  of  the  invaded  zone  failed 
to  cause  the  disease.  The  reason  for  the  fact  that  only  diseased  tissue 
from  the  marginal  fringe  of  the  invaded  zone  was  capable  of  trans- 
mitting the  disease  was  now  revealed.  In  this  portion  only,  does 
the  causal  organism  Pythiacystis  citrophthora  usually  remain  alive. 

4.  The  same  fungus  was  re-isolated  40  different  times  from  20  of 
the  cases  of  gummosis  produced  by  inoculation  and  again  found 
to  be  alive  only  at  the  outer  margin  of  the  invaded  zones,  just  as  in 
the  naturally  occurring  cases  previously  mentioned.  The  time  elapsing 
between  inoculation  and  re-isolation  was  from  1  to  12  months  in 
different  tests,  and  the  fungus  was  recovered  at  distances  of  20  inches 
or  more  from  the  original  point  of  inoculation.  One  strain  of  this 
fungus  isolated  from  a  diseased  tree  at  Whittier  was  inoculated  into 
and  re-isolated  from  three  different  trees  in  succession  during  a 
period  of  3  years  from  1912  to  1915.  During  this  time  the  fungus 
lived  in  the  bark  of  the  three  trees  for  periods  of  5,  11,  and  6  months 
respectively  and  between  these  periods  in  cultures  for  2,  2,  and  10 
months  respective^.  Transfers  from  the  original  culture  which  was 
kept  alive  for  more  than  8  years  on  cornmeal  agar  medium  were  capa- 
ble of  producing  brown  rot  of  lemon  fruits  when  tested  in  1921. 

5.  Inoculations  were  made  also  with  bits  of  lemon  fruits  affected 
with  brown  rot  and  with  the  fungus  Pythiacystis  citrophthora  isolated 
from  diseased  fruits,  with  the  same  results  as  those  obtained  by  the 
use  of  bits  of  diseased  bark  or  cultures  isolated  therefrom.  This 
experiment  served  to  show  that  the  fungus  previously  known  to  cause 
brown  rot  of  lemon  fruits  and  the  one  capable  of  inducing  this  type 
of  gummosis  were  identical.  The  detailed  experiments  on  which  these 
statements  are  based  are  being  published  in  the  Journal  of  Agricul- 
tural Research. 

RESISTANCE  OF  DIFFERENT  SPECIES  AND  VARIETIES 

Among  the  citrus  species  and  varieties  that  have  been  tested  the 
common  lemon  has  the  lowest  resistance  to  Pythiacystis  gummosis, 
and  the  sour  orange  the  highest.  The  sour  orange  usually  is  so 
resistant  to  Pythiacystis  attack  that  even  when  the  most  favorable 
conditions  are  given  by  inoculation  in  wounds,  there  is  only  a  slight 
gumming  with  rapid  healing  of  the  wounded  tissue  and  with  total 
failure  to  produce  a  diseased  lesion.  The  sour  orange  is  also  highly 
resistant  to  all  other  infectious  gum  diseases  of  importance.     Mere 


Bulletin  360]     qx.tm  DISE^VSES  OF  CITRUS  TREES  IN  CALIFORNIA 


379 


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UNIVERSITY    OF    CALIFORNIA EXPERIMENT   STATION 


gum  formation,  however,  may  be  induced  by  suitable  stimuli  in  sour 
orange  as  well  as  other  species  and  varieties.  Of  the  forms  which 
have  been  most  used  for  stocks  in  California,  the  trifoliate  orange 
probably  stands  next  to  the  sour  orange  in  resistance  and  the  sweet 
orange  next  to  the  common  lemon  in  susceptibility,  with  the  pomelo 
and  the  rough  lemon  standing  between  these  two.  Because  these  stocks 
are  grown  from  seed  there  is  a  possibility  of  variation  in  resistance 
within  each  variety  due  to  differences  between  strains  and  observations 
have  suggested  that  such  variation  actually  exists.  The  following 
observations  indicate  the  relative  resistance  of  some  of  the  common 
varieties. 

A  block  of  5,000  sweet-orange  seedlings  about  two  and  one-half 
years  old,  growing  in  nursery  rows  on  medium  heavy  clay  loam  soil, 
had  been  planted  adjacent  to  a  block  of  15,000  sour-orange  seedlings 
of  the  same  age  and  receiving  the  same  care.  All  the  trees  had  been 
irrigated  rather  frequentlj^  and  heavily.  On  October  21,  1914  four 
representative  rows  of  sweet-orange  trees  showed  the  following  per- 
centage of  infection: 


Row 

Number  of  trees  in  row 

Number  of  aflfected  trees 

Percentage  affected 

1 

222 

52 

23 

2 

213 

73 

34 

3 

212 

63 

29 

4 

180 

53 

29 

Total 

827 

241 

Average  29 

On  some  trees  only  a  small  lesion  was  evident  with  much  gum  exud- 
ing, on  others  the  bark  was  killed  to  a  distance  of  6  to  12  inches  above 
the  soil,  with  an  abundance  of  gum,  and  still  other  trees  were  dead. 
Some  trees  showed  a  strong  tendency  to  form  ridges  of  callous  tissue 
along  the  edges  of  the  dead  strips  of  bark.  A  thorough  search  in 
the  block  of  sour-orange  trees  failed  to  reveal  a  single  affected  tree. 

Differences  in  resistance  are  indicated  further  by  an  estimate 
made  by  W.  M.  Mertz,  in  a  nursery  of  citrus  seedlings  about  two  years 
old,  growing  at  the  Citrus  Experiment  Station.  The  following  is  the 
percentage  of  gum  disease  (probably  Pythiacystis  gummosis)  which 
was  recorded. 


Species 

Number  of 
trees 

Per  cent  with 
gummosis 

Citrus  Aurantium  (sour  orange) 

1000 
1000 
1000 
2000 

0.3 

Poncirus  trifoliata  (trifoliate  orange) 

1.0 

Citrus  grandis  (pomelo) 

2.5 

Citrus  sinensis  (sweet  orange) 

10.00 

Bulletin  360]    qum  DISEASES  OF  citrus  trees  in  California  381 

COXDITIOXS  FACILITATING  INFECTION  AND  DEVELOPMENT  OF 

THE  DISEASE 

A  study  of  the  external  conditions  under  which  Pythiacystis  gum- 
mosis  is  most  likely  to  become  severe,  taken  in  conjunction  with  experi- 
ments with  the  fungus,  indicates  that  the  important  factors  are:  (1) 
abundant  moisture  in  the  soil  in  contact  with  the  bark  over  a  suffi- 
ciently long  period,  (2)  favorable  soil  and  air  temperature,  and  (3) 
wounds  or  other  injuries  to  the  bark. 

Injuries,  however,  although  aids  to  infection,  are  not  necessary  if 
the  two  first-named  conditions  are  simultaneously  fulfilled.  Injuries, 
especially  below  the  soil  surface,  aside  from  inducing  infection  more 
readily  when  the  conditions  of  moisture  and  temperature  are  suitable, 
may  even  aid  in  bringing  about  infection  where  the  soil  moisture  is 
not  excessive  by  conveying  the  organism  into  the  moist  inner  bark 
tissue  where  penetration  can  start  without  the  addition  of  further 
moisture.  Wounds  or  injuries,  therefore,  while  not  absolutely  neces- 
sary for  the  entrance  of  the  parasite  when  sufficient  moisture  is  present 
are  probably  the  only  means  of  entrance  when  the  moisture  content 
of  the  soil  immediatelv  in  contact  with  the  bark  surfaces  is  not  favor- 
able  for  infection. 

Moisture. — The  severe  form  of  Pythiacystis  gummosis  is  most 
frequently  found  in  California  on  lemon  trees  where  they  are  budded 
low  on  sweet-orange  stocks  which  are  growing  on  heavy  soils  and  is 
especiallj^  prevalent  in  the  coastal  sections.  In  the  light  of  what 
has  been  stated  previously  it  is  readily  seen  why  this  should  be  true. 
Heavy  clay  soils,  being  much  more  retentive  of  moisture  than  lighter, 
more  open  soils,  furnish  the  means  by  which  water  films  may  remain 
adjacent  to  the  bark  for  periods  of  time  sufficient  to  permit  infection 
by  the  fungous  parasite. 

The  great  prevalence  of  fogs,  and  the  comparatively  slight  fluctu- 
ations of  temperature  in  the  coastal  sections  also  decrease  the  rate 
of  evaporation  in  the  soil  adjacent  to  the  trees  that  has  become  exces- 
sively wet  during  irrigation  or  rain.  In  the  interior  valleys  even  the 
same  type  of  soil  usually  drys  out  more  quickly  after  irrigations  and 
rains.  If  the  drying  out  is  rapid  there  is  not  sufficient  time  for 
infection;  if  drying  is  slow,  however,  the  time  may  be  sufficient  for 
the  penetration  of  the  fungous  hyphae.  The  effect  of  a  low  bud  union 
has  already  been  discussed.  The  lemon  bark  above  the  union  may  be 
infected  directly  when  the  fungous  parasite  has  communication  directly 
with  it  through  excessively  moist  soil  or  by  splashing  of  raindrops, 
especially  when  wounds  are  present.    High  budded  lemon  trees  have 


382  UNIVERSITY    OF    CALIFORNIA EXPERIMENT   STATION 

less  chance  of  contracting  the  disease.  If  budded  high  on  sweet-orange 
stocks  they  will  have  somewhat  more  chance  to  escape,  because  this 
stock  is  somewhat  more  resistant  than  lemon ;  it  is  nevertheless  suscep- 
tible to  infection,  especially  under  conditions  highly  favorable  to  the 
disease.  If  budded  high  on  sour-orange  stock,  however,  there  is 
practically  no  danger  of  the  disease  even  under  bad  water  conditions, 
because  of  the  great  resistance  of  the  sour-orange  stock. 

Temperature. — ^Within  certain  limits  temperature  appears  to  be 
another  important  factor,  especially  in  the  growth  of  the  fungus  and 
its  formation  of  spores  and  in  the  infection  of  the  host  as  a  result  of 
the  germination  of  these  spores.  As  has  been  previously  shown,^^  a 
temperatue  somewhere  near  25°C.  (77°F.)  appears  to  be  about  the 
optimum  for  a  sustained  growth  of  the  fungous  hyphae  in  cultures,  and 
very  little  growth  takes  place  above  30°C.  (86°F.).  If  as  seems 
probable  similar  relations  hold  for  the  growth  of  the  fungus  in  the 
bark,  we  have  an  explanation  of  the  fact  that  the  disease  progresses 
slowly  during  hot  periods,  and  also  for  the  fact  that  its  progress  is 
sometimes  completely  checked,  the  fungus  being  killed  out  in  many 
instances.  This  is  especially  true  in  the  interior  valleys,  in  the  case 
of  parts  of  lesions  entirely  above  the  soil  surface.  Here  the  temperature 
of  the  bark  frequently  becomes  many  degrees  above  the  death  point  of 
the  fungus.  Recent  experiments^*^  have  shown  that  a  temperature  of 
115 °F.  for  one  minute  is  sufficient  to  kill  the  spores  of  this  fungus  on 
infected  fruits  and  temperatures  much  lower  than  this  for  greater 
lengths  of  time  will  undoubtedly  have  the  same  effect. 

In  line  with  this  possibility  is  the  observation  that  the  rapid  en- 
largement of  invaded  areas  usually  continues  much  later  in  the  year 
or  may  proceed  throughout  the  whole  year  in  the  cooler  coastal 
sections,  while  the  activity  and  progress  of  the  disease  are  usually 
limited  to  the  winter,  spring  and  early  summer  in  the  hotter  interior 
sections.  The  following  experiment  may  be  given  as  having  a  bearing 
on  the  influence  of  temperature. 

On  September  10,  1914,  at  Whittier,  inoculation  was  made  by 
inserting  a  bit  of  mycelium  of  P.  citrophthora  into  a  cut  in  the  trunk 
of  a  five-year-old  lemon  tree.  On  October  9,  about  one  month  later, 
the  bark  was  killed  vertically  6  inches  above  and  6  inches  below 
the  ino  ulated  cut  and  about  3  inches  horizontally  at  the  widest  place, 
with  abundant  gum  on  the  surface.  On  cutting  off  the  bark  at  this 
time  it  was  noted  that  there  were  concentric  rings  at  the  cambium 
layer,  corresponding  in  number  approximately  to  the  number  of  days 

10  Fawcett,  H.  S.,  The  temperature  relations  of  growth  in  certain  parasitic 
fungi.,  Univ.  Calif.  Publ.  Agr.  Sci.,  vol.  4,  pp.  183-232,  1921. 

20  Fawcett,  II.  S.,  Calif.  Citrograph,  vol.  7,  pp.  233  and  254,  May,  1922. 


Bulletin  360]     qxJM  DISEASES  OF  CITRUS  TREES  IN  CALIFORNIA  383 

from  time  of  inoculation  and  probably  due  to  differences  in  rates  of 
advance  during  night  and  day.  The  rate  of  advance  downward  and 
upward  averaged  about  M  of  an  inch  per  day.  The  average  daily  mean 
temperature  at  the  nearest  weather  bureau  station  (Los  Angeles) 
was  about  19.5°C.  (67°F.)  with  an  average  fluctuation  of  about  5°C. 
(16°F.).  It  is  of  interest  that  the  maintained  temperatue  at  which  P. 
citrophthora  advanced  at  approximately  this  rate  in  culture  media 
was  21°C.  (71°F.)  (See  table  1,  Univ.  Calif.  Publ.  Agr.  Sci.,  vol.  4, 
p.  201).  Further  investigation  is  needed  to  determine  the  relation  of 
temperature  to  the  rapidity  of  advance  of  the  fungus  in  invaded 
tissue. 

Other  Conditions. — A  further  condition  which  contributes  to  the 
chance  of  infection  in  the  orchard  is  that  of  deep  planting,  or  the 
accumulation  of  soil  next  to  the  trunk  of  the  trees.  Under  these 
conditions  there  is  greater  surface  of  the  bark  in  contact  with  the  soil, 
and  the  bud  union,  if  low,  becomes  buried.  The  bark  over  the  union 
between  stock  and  scion  is  usually  especially  susceptible  to  infection, 
probably  because  of  frequent  irregularities  of  growth,  suckering,  etc. 
The  practice  of  loosening  the  soil  by  digging  about  the  base  of  the 
tree,  if  no  injuries  are  produced,  probably  helps  to  prevent  infection 
by  hastening  the  drying  out  of  the  soil.  This  practice  may,  on  the 
other  hand,  if  carelessly  performed,  result  in  injuries  to  the  bark 
and  aid  in  the  entrance  of  the  organism  or  actually  introduce  it  into 
the  inner  bark  tissues  on  bits  of  soil.  Any  agencies  such  as  gophers, 
field  mice,  or  rabbits,  or  tools  used  in  orchard  operations  that  produce 
injuries  to  the  bark,  may  contribute  to  the  ease  of  infection  by  the 
fungus  parasite.  Infection,  however,  appears  to  take  place  frequently 
through  the  sound  uninjured  bark. 

The  following  is  a  concrete  example  of  a  set  of  conditions  that 
resulted  in  a  serious  outbreak  in  a  5-acre  6-year-old  lemon  orchard  at 
Whittier  in  1914,  on  medium  heavy  clay  soil. 

The  season  had  been  one  of  unusually  heavy  rainfall.  A  volunteer 
crop  of  barley  was  cut  while  green  in  March  and  thrown  around  the 
trees  in  contact  with  their  trunks.  This  operation  was  followed  by 
heavy  rains  and  cloudy  days.  The  following  May  90  trees  with 
brown-rot  gummosis  in  various  stages  were  discovered.  The  lesions 
appeared  to  have  started  at  various  places  below  and  above  the  surface 
of  the  soil.  Some  of  those  above  the  surface  had  started  at  places 
where  water  sprouts  had  been  broken  off,  but  many  appeared  to  have 
started  in  sound,  uninjured  bark.  The  barley  piled  around  the  trunks 
had  undoubtedly  aided  in  retaining  films  of  water  for  a  sufficient  time 
to  permit  germination  of  the  spores  and  the  penetration  of  the  hyphae. 


384  UNIVERSITY    OP    CALIFORNIA EXPERIMENT   STATION 

All  the  diseased  trees  were  successfully  treated  under  the  author's 
directions  and  the  results  will  be  discussed  under  the  heading  of 
control. 

FUSARIUM  AS  A  SECONDARY  AID  IN  THE  DEVELOPMENT  OF 

PYTHIACYSTIS  GUMMOSIS 

During  the  examination  of  a  large  number  of  naturally  occurring 
cases  of  Phythiacystis  gummosis  it  was  noticed  that  a  species  of 
Fusarium  frequently  accompanied  and  was  closely  associated  with  P. 
citropJithora  in  the  diseased  tissue.  The  question  arose  as  to  whether 
the  Fusarium  played  any  part  in  the  development  or  the  severity  of 
the  disease. 

Fusarium  has  been  mentioned  frequently  in  the  literature  as  hav- 
ing some  possible  relation  to  certain  types  of  gum  disease.  Briosi^^ 
and  McAlpin-^  concluded  that  Fusarium  limoni  Briosi  played  an  im- 
portant part  in  mal  di  gomma  in  Italy  and  in  Australia.  Earle  and 
Rogers,^^  though  not  able  to  produce  gummosis  by  inoculation  with 
Fusarium,  believed  that  under  certain  conditions  it  was  probably  a 
factor  in  a  certain  type  of  gum  disease  in  Cuba.  The  present  writer 
also  had  previously  found  species  of  Fusarium  repeatedly  associated 
with  mal  di  gomma  or  footrot  in  Florida,  but  inoculations  w^ith  them 
had  given  negative  results. 

Although  the  experiments  made  with  this  question  in  mind  have 
been  too  few  as  yet  to  allow  definite  conclusions  to  be  drawn,  certain 
results  have  indicated  that  the  severity  of  the  disease  is  slightly 
increased  by  adding  Fusarium  along  with  Pythiacystis  citrophthora 
at  the  time  of  inoculation.  The  characteristics  of  the  disease,  however, 
were  the  same  as  when  the  Pythiacystis  was  inserted  alone,  while 
inoculation  with  Fusarium  alone  failed  to  produce  this  type  of  gum- 
mosis. Fusarium  inoculations  caused  only  a  slight  killing  of  a  narrow 
layer  of  tissue  along  the  cut  without  gumming  and  later  resulted  in  a 
cracking  of  outer  layers  of  bark  around  the  cut.  Otherwise  the  effect 
was  not  different  in  either  case  from  that  produced  in  the  inoculated 
cuts  used  as  checks  on  the  same  trees.  The  details  of  this  experiment 
will  be  found  elsewhere  in  this  article. 


21  Briosi,  G.,  Intorno  al  mal  di  gomma  degli  agrumi  (Fusisporium  limoni, 
Briosi).  Atti  R.  Acad.  Lincei,  Roma,  ser.  3^,  vol.  2;  Memoria  della  classe  di 
science  fisiche  etc.,  pp.  485-496,  1878. 

22  McAlpine,  D.,  Fungus  diseases  of  citrus  trees  in  Australia  and  their 
treatment,  Melbourne,  p.  132,  1899. 

23  Earle,  F.  S.  and  Rogers,  J.  M.,  *' Citrus  diseases  at  San  Pedro  in  1915,** 
in  San  Pedro,  Isle  of  Pines,  Citrus  Path.  Lab.,  Ann.  Report  1,  pp.  36-38,  1915. 


Bulletin  360]     qUM  DISEASES  OF  CITRUS  TREES  IN  CALIFORNIA  385 


METHODS  OF  CONTEOL 

Prevention. — The  causative  fungus  usually  infects  the  bark  at  or 
below  the  surface  of  the  soil.  The  roots  of  a  susceptible  variety  are 
more  resistant  than  the  bark  of  the  trunk.  As  pointed  out  above, 
some  of  the  principal  conditions  of  infection  are:  (1)  excessive  wet 
soil  in  contact  with  the  trunk,  as  a  result  of  improper  irrigation  and 
drainage,  or  long  periods  of  continuous  rains;  (2)  deep  planting,  or 
soil  piled  up  around  the  trunk,  especially  when  it  is  highly  retentive  of 
moisture;  (3)  injuries  to  the  bark  at  the  base  of  the  tree,  especially  in 
wet  weather. 

The  methods  which  have  been  found  effective  and  are  now  in  com- 
mon use  in  preventing  infection  brought  about  by  these  conditions, 
are  as  follows : 

The  soil  is  pulled  back  from  the  base  of  the  tree  to  expose  the  top 
of  the  first  main  roots  and  is  left  in  a  circular  ridge  to  exclude  irriga- 
tion water  from  the  depression  next  to  the  trunk.  If  the  depression 
occasionally  becomes  filled  up  with  water,  the  resulting  condition  is 
usually  not  so  serious  as  when  the  soil  is  against  the  bark,  because 
the  water  after  a  rain  soon  percolates  into  the  soil.  Under  usual 
weather  conditions  in  California,  with  most  citrus  soils,  the  length  of 
time  that  water  would  stand  in  such  a  depression  is  not  sufficient  to 
produce  infection. 

This  pulling  away  of  the  soil  is  most  important  on  the  heavier  clay 
soils,  while  on  looser  sandy  soils  this  practice  is  not  so  necessary. 

As  an  additional  means  of  prevention  the  bark  at  the  base  of  the 
trunk  is  painted  with  Bordeaux  paste  or  other  non-injurious  fungicide 
after  pulling  the  soil  away. 

Experiments  in  Prevention. — The  following  experiment  on  15- 
year-old  lemon  trees  at  Chula  Vista  has  been  reported  by  Prizer.^* 
The  trees  had  low  bud  unions  and  the  soil  (a  heavy  ''adobe")  had 
not  been  carefully  pulled  back  from  the  base.  Ten  per  cent  of  the 
trees  had  new  lesions  on  the  trunk  the  year  before.  Thirty-six  per  cent 
of  the  trees  had  already  been  killed  by  the  disease  and  were  missing. 
The  trunks,  in  two  rows  containing  81  trees,  were  painted  with  Bor- 
deaux paste  in  the  fall  of  1913,  and  two  other  rows  next  to  them 
containing  72  trees  were  left  unpainted  as  controls.  During  the  next 
summer  there  were  6.9  per  cent  of  the  unpainted  trees  affected  with 
new  lesions  as  compared  with  1.2  per  cent  of  the  painted. 


24Prizer,  J.  A.,  Mouth.  Bull.  Calif.  State  Comm.  of  Hort.,  vol.  4,  pp.  7-19, 
1915. 


386 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT   STATION 


In  1913  and  1914  the  San  Diego  Fruit  Company,  Cliula  Vista, 
California,  instituted  methods  of  prevention.  Greater  care  was  used  in 
keeping  the  soil  away  from  the  base  of  the  tree.  During  the  summer 
and  fall  of  1913  most  of  the  trunks  of  the  trees  were  painted  with 
Bordeaux.  The  following  table  shows  the  number  of  new  cases  in  6 
orchards  on  which  a  three-year  record  was  available. 


Acres  in  orchard 

Number  of  trees  newly  infected 

1912 

1913 

1914 

10 

20 

7 

0 

30 

9 

25 

5 

30 

19 

5 

5 

10 

11 

4 

0 

20 

43 

43 

4 

10 

Total     110 

21 

123 

5 

89 

2 

16 

The  marked  decrease  in  the  number  of  new  cases  of  gummosis 
from  1912  to  1914  will  be  noted.  The  total  number,  123  in  1912,  had 
decreased  to  16  or  about  one-eighth  as  many  in  1914. 

At  Santa  Paula  records  of  trees  affected  with  gummosis  during 
1912  to  1914  inclusive,  were  preventive  treatment  somewhat  similar  to 
that  carried  out  at  Chula  Vista  was  applied,  show  a  similar  decrease  or 
falling  off  in  new  cases.*  These  records  include  both  Botrytis  gum- 
mosis and  pythiacystis  gummosis  and  possibly  some  other  forms  of 
gummosis  since  these  were  not  separated.  The  following  table  of 
various  blocks  gives  the  numbers  of  the  trees  with  new  lesions. 

Table  1 


Number  of  trees  in 
block 

Number  of  trees  newly  infected 

Block 

1912 

1913 

1914 

A 

1213 

33 

14 

4 

B 

2588 

150 

132 

5 

C 

1340 

124 

28 

4 

D 

685 

29 

9 

9 

E 

575 

9 

0 

6 

F 

2925 

103 

31 

0 

G 

2862 

51 

58 

22 

H 

3368 

62 

62 

30 

I 

4538 

154 

99 

30 

J 

1570 

13 

727 

26 
459 

0 

21664 

110 

*  The  author  is  indebted  to  the  Limoneira  Company  through  J.  D.  Culbert- 
son,  for  the  use  of  these  records. 


Bulletin  360]     gum  DISEASES  OF  CITRUS  TREES  IN  CALIFORNIA 


387 


This  record  shows  that  in  two  years  the  total  number  of  newly 
infected  trees  decreased  from  3.3  to  0.5  per  cent  and  this  decrease  was 
apparently  due  to  the  adoption  of  preventive  measures  based  on  a 
knowledge  of  the  real  nature  of  the  disease,  gained  from  the  experi- 
mental results  described  in  previous  sections  of  this  paper. 

The  following  data  on  the  use  of  Bordeaux  were  also  taken  from 
the  records  kept  by  the  Limoneira  Company.  In  one  orchard  all  trees 
with  the  exception  of  8  rows  of  about  35  trees  each,  were  sprayed  with 
Bordeaux  mixture  two  years  in  succession,  from  1910  to  1911,  and 
following  this  the  surface  of  the  ground  and  lower  branches  of  the 
trees  were  sprayed  each  fall  during  1912  and  1913.  The  records  show 
the  number  of  new  developments  of  gummosis  in  1914.  Each  set  of  8 
rows  contained  approximately  280  trees. 


Number  of  trees 

Trees  with  lesions  for 
first  time 

Trees  newlv  gumming 
at  old  lesions 

Total 

Number 

Per  cent 

Number 

Per  cent 

Per  cent 

Unsprayed  280 

10 
3 

2.8 
.9 

49 

24 

17.5 

8.1 

20.3 

Sprayed  280 

9.0 

It  is  seen  that  the  unsprayed  trees  showed  2.8  per  cent  of  newly 
infected  trees,  or  somewhat  the  same  as  the  average  for  21664  trees  in 
the  same  locality  in  1912  before  any  preventive  treatments  were 
started  (table  1).  The  sprayed  trees,  however,  showed  only  0.9  per 
cent  newly  infected,  or  about  one-third  as  many.  The  continued 
development  of  the  disease  on  old  lesions  was  17.5  per  cent  on  un- 
sprayed trees  as  against  8.1  per  cent  on  sprayed  trees. 

An  effective  means  of  prevention  for  new  plantings,  and  one  es- 
pecially desirable  on  heavy  soils,  is  to  use  trees  budded  high,  1  to  2 
feet  or  more,  on  resistant  stocks,  such  as  the  sour-orange.  The  desira- 
bility of  this  stock  because  of  its  resistance,  not  only  to  Pythiacystis 
gummosis  but  also  to  psorosis  and  other  gum  diseases,  has  become  so 
well  recognized  that  a  greater  part  of  the  new  plantings  in  California 
are  of  trees  having  this  stock. 

Treatment. — Even  when  careful  attention  is  given  to  prevention,  a 
few  cases  of  Pythiacystis  gummosis  will  often  occur  from  time  to 
time,  especially  with  trees  on  susceptible  stocks  planted  in  hea\y  soils. 
The  following  method  of  treatment  has  been  adopted  as  the  result  of 
much  experimental  work  in  which  the  cooperation  of  citrus  growers 
has  been  of  great  assistance.^^    The  extent  to  which  the  bark  is  killed 

25  Fawcett,  H.  S.,  Calif.  State  Comm.  of  Hort.  Monthly  Bull.  2,  pp.  601-617, 
1913,  and  Prizer,  J.  A.,  Calif.  State  Comm.  of  Hort.,  Month.  Bull.  4,  pp. 
7-19,  1915. 


388 


UNIVERSITY    OF    CALIFORNIA — EXPERIMENT   STATION 


through  to  the  wood  is  first  ascertained  by  scraping  slightly.  Then 
the  brownish  killed  bark  (invaded  zone)  is  dissected  out  with  a  heavy 
knife,  cutting  through  to  the  wood  about  l^  inch  beyond  the  invaded 
zone  on  the  sides  and  1  to  2  inches  beyond  at  the  top  and  bottom 
in  a  manner  similar  to  that  shown  in  figure  4&.  The  cuts  on  the  two 
sides  are  ususally  brought  together  above  and  below,  making  an  acute 
angle.  No  attempt  is  made  to  cut  beyond  the  outer  gummous  zone, 
since  it  has  been  shown  that  this  zone  does  not  contain  the  invading 
parasite.  The  live  bark  over  this  zone  of  gummous  influence,  not  yet 
invaded  by  Pythiacystis  citrophthora,  will  usually  recover  rapidly  as 


Fig.  4. — Method  of  cutting  away  diseased  bark  in  treatment  of  a  severe 
case  of  Pythiacystis  gummosis. 

(a)  Result  of  inoculation  with  a  bit  of  diseased  bark  on  November  16, 
1912.     Photographed   September   3,   1913.     Invaded   area   3x8.5   inches. 

(6)  Bark  cut  away  September  3,  1913.  Ready  for  painting  with  Bordeaux 
paste. 

(c)  Same  tree  on  March  17,  1916,  (314  years  later)  showing  growth  at 
edges  of  wound  and  asphalt  paint  on  exposed  wood. 


soon  as  the  invaded  portion  has  been  removed  and  the  further  progress 
of  the  fungus  has  been  stopped.  These  cut-out  places  and  the  entire 
trunk  are  then  painted  with  Bordeaux  paste  (1  lb.  copper  sulphate 
and  2  lbs.  rock  lime  to  l^/^  gallons  of  water)  or  some  other  suitable 
fungicide  (see  formulae  under  "Psorosis").  The  fungicides  other 
than  Bordeaux  paste  most  generally  used  in  California  are  coal  tar 
products  from  which  the  lighter  oils  have  been  distilled.  Arrow  Car- 
bolineum  and  Creolineum  XXX  are  trade  names  of  some  of  those  in 
common  use. 


Bulletin  360]     gum  DISEASES  OF  CITRUS  TREES  IN  CALIFORNIA 


389 


The  elimination  of  the  tissue  invaded  by  the  causative  organism  is 
the  most  important  feature  of  the  treatment,  the  purpose  of  the  fungi- 
cide being  to  prevent  reinfection,  or  to  kill  the  organism  in  small  bits 
of  tissue  left  behind. 

After  treatment  the  gum  usually  continues  to  exude  for  some  time 
at  the  edges  of  the  cut-out  areas.  This  is  left  undisturbed  except  to 
examine  the  edges  of  the  cuts  to  ascertain  whether  any  bark  is  being 


Fig.  5.^(a)  Young  lemon  tree  into  which  a  sour-orange  seedling  i/4  inch  in 
diameter  had  been  inarched  over  a  gummosis  lesion  about  2%  years  before.  The 
inarched  seeding  had  increased  to  %  inch  in  diameter  and  had  caused  a  ridge 
of  growth  above  as  shown  alongside  the  white  chalk  mark. 

(fo)  Old  lemon  tree  showing  gummosis  lesion  bridged  by  inarching  sprouts 
from  .the  base.     About  5  years  after  inarching. 


killed  by  further  invasion  of  the  fungus.  The  gum  appears  to  act  as  a 
protective  covering  under  which  callous  tissue  is  readily  formed  along 
the  edges  of  the  cuts.  After  these  cut  edges  begin  to  heal,  the  exposed 
wood  is  painted  with  asphalt  paint  or  other  good  covering  (fig.  4c). 
In  cases  where  a  large  part  of  bark  on  the  trunk  has  been  killed 
by  the  rapid  invasion  of  Pythiacystis  citrophthora,  so  that  the  trunk 


390  UNIVERSITY    OF    CALIFORNIA— EXPERIMENT   STATION 

is  girdled  or  nearly  so,  bridge  grafting^^  and  inarching^^'  ^^'  ^^'  may 
often  be  resorted  to  in  order  to  save  the  tree  (fig.  5). 

The  advisability  of  this  practice,  in  any  particular  instance,  will 
depend  upon  the  age  and  condition  of  the  tree  and  other  factors. 
A  modification  of  the  usual  method  of  bridge  grafting  has  been  success- 
ful with  citrus.  Instead  of  a  shoot  or  branch  being  grafted  into  the 
bark  both  above  and  below  the  injured  portion,  a  sprout  at  or  below 
the  lower  edge  of  the  wound  is  allowed  to  grow.  When  of  sufficient  size 
the  top  part  is  cut  off,  bent  over,  and  grafted  into  the  bark  above  the 
diseased  area  (fig.  5&). 

Another  similar  method  of  saving  or  aiding  an  injured  tree  is 
inarching,  in  which  one  or  more  small  seedling  trees  are  planted  close 
to  the  trunk  and  the  top  grafted  into  the  bark  above  the  injury. 
Where  quick  relief  is  necessary  and  suitable  sprouts  cannot  be  stimu- 
lated, this  method  has  proved  beneficial  (fig.  5a). 

Experiments  in  Treatment.-— At  Santa  Paula,  through  J.  D. 
Culbertson  of  the  Limoneira  Company,  and  at  Chula  Vista,  through 
J.  A.  Prizer  of  the  San  Diego  Fruit  Company,  experiments  planned  in 
consultation  with  the  author  were  carried  out. 

Previous  to  these  experiments  a  large  number  of  methods  of  treat- 
ment were  tested,  especially  the  method  of  slitting  the  bark  and  apply- 
ing neat's  foot  oil,  all  of  which  gave  unsatisfactory  results  except 
with  trees  treated  when  the  lesions  were  very  small.  As  soon  as  the 
investigation  began  to  indicate  that  the  disease  was  due  to  some 
infection  the  application  of  Bordeaux  paste  was  used,  first  in  connec- 
tion with  slitting  (fig.  6)  and  later  in  connection  with  cutting  away 
of  the  killed  tissue  of  the  lesion  (fig.  4&). 

Since  a  detailed  report  of  these  experiments  at  Chula  Vista  has 
been  published  by  Prizer,^^  only  a  brief  summary  of  the  results  will  be 
included  here. 

In  one  orchard  alternate  rows  were  left  without  the  Bordeaux  as 
checks,  but  received  the  same  treatment  as  to  cutting  away  and  slitting. 
The  lesions  were  then  divided  into  three  classes  called  slight,  medium 
and  bad.    The  results  of  the  first  year's  treatment  (1912)  before  the 


26  Paddock,  E.  H.,  Bridge  grafting  on  Citrus.  Month.  Bull.  Calif.  State 
Comm.  Hort.,  vol.  81,  pp.  72-73,  1919. 

27  Rundle,  H.  E.,  Inarching  Citrus  Trees.  Calif.  Cultivator,  vol.  44,  pp. 
540-541,  1916. 

28  Hume,  H.  H.,  Citrus  fruits  and  their  culture,  ed.  4,  196  pp.,  New  York, 
1911. 

20  Fawcett,  H.  S.,  Experiments  in  Bridge  Grafting  and  Inarching  in  Con- 
nection with  Gummosis  of  Citrus.    Calif.  Citrograph,  vol.  8,  pp.  68  and  95,  1923, 

80  Prizer,  J.  A.,  Calif.  State.  Comm.  of  Hort.,  Month.  Bull.,  vol.  4,  pp. 
7-19,  1915. 


Bulletin  360]     qUM  DISEASES  OF  CITRUS  TREES  IN  CALIFORNIA 


391 


method  of  cutting  out  was  well  perfected  (it  not  yet  being  known  how 
far  cutting  away  was  necessary)  were  as  follows: 


Check  (Bordeaux  not  used) 

Cured 

Still  gumming 

Trees  dead 

Per  cent 

Per  cent 

Per  cent 

SHght 

81.2 

18.7 

0 

Medium 

50.0 

50.0 

0 

Bad 

7.1 

21.2 

71.5 

Bordeaux  used 

Slight 

82.3 

17.6 

0 

Medium 

62.5 

37.0 

0 

Bad 

23.0 

66.0 

11.0 

It  is  to  be  noted  that  the  difference  in  cured  lesions  in  favor  of  the 
application  of  Bordeaux  after  cutting  is  not  large  in  the  slight  and 
medium  classes.  In  the  bad  class,  however,  the  proportion  of  cures  is 
23  per  cent  for  the  Bordeaux  against  7.1  per  cent  for  the  check,  while 
the  dead  trees  in  this  class  are  only  11  per  cent  for  the  Bordeaux 
against  71.5  per  cent  for  the  check. 

The  results  for  a  period  of  three  years,  1912-1914  inclusive  as 
obtained  in  treating  209  new  lesions  on  trees  which  had  not  been 
infected  before,  are  of  interest. 


Stage  of  disease 

Trees  cured 

Trees  still  gumming 

Trees  dead 

Number 

Per  cent 

Number 

Per  cent 

Number 

Per  cent 

Slight      

115 
37 
35 

95 

80.4 
83.3 

5 

7 
2 

4.1 

15.2 

4.7 

1 

2 
5 

.9 

Medium 

4.3 

Bad 

11.9 

The  results  show  that  although  a  large  percentage  of  the  trees  in 
the  bad  classes  may  be  cured,  a  still  larger  percentage  of  those  in  the 
slight  class  yield  to  treatment.  It  is  important,  therefore,  to  inspect 
all  trees  sufficiently  often  to  detect  as  far  as  possible  all  cases  when 
slight,  not  only  because  of  the  decrease  in  loss  of  trees  themselves, 
but  to  prevent  severe  injury  from  loss  of  bark.  Prizer  considers  it 
necessary  in  the  case  of  heavy  soils  near  the  coast  to  make  not  less 
than  three  separate  tree-to-tree  inspections  a  year,  and  possibly  four 
on  the  worst  orchards. 

In  the  Limoneira  Company  orchards  at  Santa  Paula,  similar  meth- 
ods of  treatment  were  carried  out  with  satisfactory  results  as  far  as 
Pythiacystis  gummosis  was  concerned.     The  Botrytis  gummosis  was 


392  UNIVERSITY    OF    CALIFORNIA — EXPERIMENT   STATION 

more  prevalent  in  these  orchards  than  in  the  Chula  Vista  orchard  and 
because  of  the  different  character  of  the  disease  a  method  of  scraping 
off  the  outer  cortical  layers,  as  well  as  cutting  out  entirely  to  the  wood 
was  worked  out.     It  is  described  more  fully  under  that  disease. 

Later  the  Limoneira  Company  desired  to  displace  the  Bordeaux, 
if  possible  by  some  other  fungicide,  partly  because  the  presence  of 
Bordeaux  when  followed  by  fumigation  with  hydrocyanic  acid  gas 
was  thought,  under  the  climatic  conditions  at  Santa  Paula,  to  empha- 
size the  injury  to  the  foliage.  The  writer  was  asked  to  suggest  some 
substitutes  for  trial,  and  the  following  materials  were  tried  out  on 
6-year-old  trees  by  the  Limoneira  Company:  (1)  Carbolic  acid  (5%) 
1  part,  and  liquid  whale-oil  soap,  1  part,  on  2  trees;  (2)  Avenarius 
carbolineum,  1  part,  emulsified  with  3  parts  of  liquid  whale-oil  soap, 
on  2  trees;  (3)  corrosive  sublimate,  1  gram  in  1000  c.c.  of  denatured 
alcohol,  on  1  tree;  (4)  Rex  limesulfur  solution,  1  part  to  1  part  of  a 
paste  of  lime,  to  give  it  body,  on  2  trees;  (5)  Bordeaux  paste  ( 1-2-1 1/^) 
on  1  tree;  (6)  No  application,  as  control,  4  trees. 

These  trees  were  examined  from  time  to  time  for  two  years  and  no 
marked  difference  in  the  recovery  of  any  of  them  that  could  be  attrib- 
uted to  the  application  of  one  substance  over  another,  could  be  made 
out.  Avenarius  carbolineum  full  strength  and  corrosive  sublimate,  1 
part  to  1000  parts  of  denatured  alcohol,  and  lime-sulfur  solution 
diluted  with  equal  parts  of  water,  were  used  in  the  same  way  at 
Whittier  in  September,  1915,  with  results  similar  to  those  at  Santa 
Paula. 

Since  all  the  trees  recovered,  even  the  checks,  it  is  indicated  that 
the  proper  dissection  of  the  diseased  bark  is  probably  more  important 
than  the  fungicide  applied  to  the  wound.  The  writer^^  had  used 
Avenarius  carbolineum  emulsified  with  equal  parts  of  soap  and  water 
in  Florida  for  certain  diseases  of  orange  trees.  In  1916  C.  C.  Miller, 
who  had  used  Avenarius  carbolineum  at  full  strength  without  injury, 
on  deciduous  trees,  began  using  this  substance  as  a  substitute  for 
Bordeaux.  Miller 's^^  first  report  of  this  treatment  is  likely  to  be 
misleading  because  he  was  not  then  familiar  with  the  difference  be- 
tween the  different  types  of  citrus  gummosis  and  because  he  conducted 
no  extensive,  well  checked,  comparative  experiments  comparing  carbol- 
ineum and  other  substances.  It  is  now  indicated  that  any  good  fungi- 
cide that  is  not  injurious  to  the  bark  is  efficient  for  use  in  the  treat- 
ment of  this  disease. 


31  Fawcett,  H.  S.,  Florida  Agr.  Exp.  Sta.  Bull.  106,  1911. 

32  Miller,    C.    C,    Month.    Bull.    Calif.    State    Comm.    of    Hort.,    vol,    7,    pp. 
488^93,  1918. 


Bulletin  360]     gUM  DISEASES  OF  CITRUS  TREES  IN  CALIFORNIA 


893 


An  example  of  what  can  be  done  in  controlling  Pythiacystis  gum- 
mosis  when  it  is  treated  promptly  and  with  care,  is  furnished  by  a 
five-acre  lemon  orchard  at  Whittier  on  clay  loam  soil,  previously  re- 


Fig.  6. — Showing  a  method  formerly  used  in  treating  brown-rot  gummosis 
by  cutting  vertical  slits  in  the  bark,  one  on  each  side  of  the  diseased  lesion 
and  several  down  through  the  invaded  area.  This  treatment  appeared  to  be  of 
some  value  for  mild  cases,  but  was  useless  for  more  severe  outbreaks. 


f erred  to,  where  conditions  facilitating  infection  obtained.  About  20 
per  cent  of  the  6-year-old  trees  became  infected  after  a  period  of 
heavy  rains  following  the  application  of  green,  freshly  cut  barley 
about  the  trunks  in  March,  1914. 


394  UNIVERSITY    OF    CALIFORNIA — EXPERIMENT   STATION 

The  lesions  noticed  in  May  varied  in  size  from  those  just  started 
to  lesions  extending  one-half  to  two-thirds  around  the  circumference 
of  the  bark  on  the  trunk.  On  May  25,  1914,  the  trees  were  treated  by 
dissecting  out  all  the  brown  killed  bark  (invaded  zone)  and  cutting 
about  %  inch  beyond  this  invaded  zone  on  the  sides  and  1  to  2  inches 
beyond  at  the  top  and  bottom,  as  is  shown  in  fig.  4&.  The  cuts  on  the 
two  sides  were  usually  brought  together  above  and  below,  making  an 
acute  angle. 

No  attempt  was  made  to  cut  beyond  the  outer  gummosis  zone  of 
gummous  influence,  so  that  in  most  cases  on  larger  lesions  this  zone 
probably  extended  into  the  remaining  bark  a  considerable  distance 
beyond  the  cuts.  As  has  been  pointed  out  in  the  description  of  the 
disease  and  elsewhere,  the  margin  of  the  zone  actually  invaded  by  the 
causal  fungus  usually  lags  far  behind  the  zone  in  which  gum  formation 
occurs.  These  cut-out  places  and  the  entire  trunks  were  painted  with 
Bordeaux  paste  (1  lb.  copper  sulfate  and  2  lbs.  rock  lime  to  1%  gallons 
water).  In  order  to  prevent  further  infection  the  soil  was  pulled 
back  from  the  trunk  as  far  as  the  top  of  the  first  main  roots. 

After  treatment  the  gum  continued  to  exude  in  considerable 
quantities  at  the  edges  of  the  cut-out  areas.  This  gum  was  left  undis- 
turbed except  for  examining  the  edges  of  the  cuts  to  ascertain  whether 
the  bark  was  being  killed  further.  The  gum  appeared  to  act  as  a 
protective  covering  under  which  callous  tissue  was  soon  seen  to  be 
forming  rapidly  along  the  edges  of  the  cuts. 

On  June  22,  1914,  about  one  month  after  treatment,  an  inspection 
showed  only  5  trees  with  new  areas  of  killed  bark,  all  of  which  areas 
were  on  trees  previously  treated.  On  August  3,  7  additional  areas 
of  killed  bark  were  found,  and  on  September  15,  8  additional  areas, 
all  except  2  of  which  were  on  trees  previously  treated. 

On  March  29,  1915,  a  rapid  healing  was  noted  at  all  the  cuts  and 
no  effect  on  the  foliage  could  be  detected.  Three  new  lesions,  all 
originating  below  the  surface  of  the  soil,  were  found. 

On  July  30,  1915,  3  trees  which  had  lost  bark  from  areas  extending 
over  two-thirds  to  three-fourths  their  trunk  circumferences,  were 
showing  a  slight  yellowing  of  the  foliage,  but  only  on  branches  directly 
above  the  dissected  areas.  By  March  6,  1916,  however,  only  one  tree 
showed  any  appreciable  effect  of  the  removal  of  bark  and  this  was  one 
from  which  about  120  square  inches  had  been  removed.  The  trunks  of 
all  the  trees  had  been  painted  again  with  Bordeaux  paste  in  the  fall 
of  1915,  and  no  new  cases  were  found  during  the  spring  and  summer 
of  1916. 


Bulletin  360]     qUM  DISEASES  OF  CITRUS  TREES  IN  CALIFORNIA 


395 


The  rapidity  of  healing  of  cut-out  places  of  various  sizes  is  indi- 
cated by  table  2.  These  figures  were  obtained  by  measuring  the  areas 
in  two  directions  and  estimating  the  number  of  square  inches  in  each. 
While  the  figures  are  only  approximate,  they  give  a  rough  idea,  at 
least,  of  the  rate  at  which  the  different  sized  areas  on  lemon  trees  of 
this  age  may  be  expected  to  close  up  with  new  bark  tissue. 

Table  2 


Number  of  areas 

Size  of  original  areas, 
May  1914 

Average  size  of  areas, 
June  1916 

Average  size  of  areas, 
April  1921 

sq.  in. 

sq.  in. 

sq.  in. 

5 

.5 

0.0 

0.0 

3 

1.0 

.08 

0.0 

8 

2.0 

.97 

0.0 

4 

3.0 

1.2 

0.0 

10 

4.0 

2.7 

0.6 

2 

5.0 

3.-1 

0.5 

5 

6.0 

3.7 

0.0 

4 

7.0 

2.5 

0.9 

7 

8.0 

4.0 

0.6 

5 

10.0 

4.2 

1.0 

2 

11.0 

8.0 

2.0 

5 

12.0 

9.8 

3.2 

3 

16.0 

11.3 

5.0 

5 

22.0 

17.0 

5.6 

4 

35.0 

27.0 

7.5 

3 

40.0 

22.0 

15.0 

7 

50.0 

42.5 

25.0 

4 

75.5 

55.0 

23.7 

5 

105.6 

69.0 

39.6 

1 

130.0 

80.0 

60.0 

It  will  be  noted  that  all  wounds  less  than  3  square  inches  in  size 
at  time  of  treatment  were  closed,  or  insignificant,  at  the  end  of  2 
years.  Nearly  all  wounds  less  than  10  square  inches  in  size  at  time  of 
treatment  were  closed  in  7  years.  Areas  larger  than  10  square  inches 
at  time  of  treatment  were  reduced  to  about  one-half  to  two-thirds  of 
their  original  size  in  2  years  and  about  one-third  to  one-fourth  their 
original  size  in  7  years. 


TEEATMENTS  FOR  GUMMOSIS  TO  BE  AVOIDED,  OR  USED  WITH  CARE 
A  treatment  for  wounded  tree  trunks,  mention  of  which  is  found  in 
very  early  literature,  is  the  use  of  fresh  cow  dung  applied  to  the 
surface  of  wounded  tissue.  Whatever  value  this  practice  may  have 
for  mechanical  wounds,  when  parasitic  organisms  happily  do  not  get 


296 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


started  in  such  a  medium,  it  is  certainly  extremely  dangerous  when 
applied  to  tree  trunk  affected  with  either  Pythiacystis  or  Botrytis 
gummosis,  for  the  reason  that  ideal  conditions  for  growth  of  the  fungus 
parasites  would  seem  to  be  furnished  by  the  presence  of  the  fresh, 
moist  cow  dung. 

An  example  of  the  results  of  such  practice  will  be  briefly  mentioned 
here. 

In  February,  1913,  on  the  advice  of  a  foreign  expert  in  citrus 
culture,  a  grower  treated  a  large  number  of  trees  that  had  previously 
had  gummosis,  by  coating  their  entire  trunks  with  cow  dung,  then 
wrapping  them  first  with  newspapers  and  over  this,  burlap.  A  bunch 
of  dry  weeds  and  grass  was  then  placed  in  the  forks  of  the  branches. 
Fifteen  pounds  of  sulfur  was  added  to  every  50  gallons  of  the  dung. 
During  the  latter  part  of  May  it  was  discovered  that  a  large  amount 
of  softening,  dying  and  decajdng  of  new  bark  was  going  on  under 
these  wraps.  Gum  pockets  were  forming  on  large  limbs  above  the 
wrappings  under  the  bunches  of  grass  in  the  forks.  All  wraps  were 
then  removed  as  fast  as  possible,  the  dung  scraped  off,  the  dead,  soft, 
decaying  patches  of  bark  cut  away  and  the  entire  trunk  painted  with 
Bordeaux  paste.  It  is  not  possible  to  say  definitely  what  would  have 
happened  to  these  trees  if  the  wraps  had  remained,  but  many  of  them 
would  probably  have  been  injured  beyond  recovery  by  the  rapid 
increase  in  the  decay  of  bark  under  the  burlap. 

Records  had  been  kept  of  1023  trees  so  treated.  Of  these  293  had 
previously  had  their  trunks  painted  with  Bordeaux  paste  in  connection 
with  gum  disease  treatment.  The  remaining  730  had  not  been  painted 
with  Bordeaux  previous  to  wrapping.  It  was  interesting,  therefore, 
on  removal  of  the  wraps,  to  compare  the  condition  of  the  trunks 
bordeauxed  before  wrapping  with  those  not  so  treated.  The  following 
data  were  obtained : 


Trees  not  Bordeauxed  before  wrapping 

Gummosis  active  on  removal  of  wraps 
Number 

Per  cent 

730 

Trees  Bordeauxed  before  wrapping 

486 
31 

66.6 

293 

10.6 

These  results  seem  to  show  that  the  presence  of  the  Bordeaux  under 
the  dung  at  time  of  application  prevented  a  greater  part  of  the 
damage  in  the  case  of  the  trees  so  treated. 


Bulletin  360]     g^JM  DISEASES  OF  CITRUS  TREES  IN  CALIFORNIA  397 

Another  treatment  which  frequently  results  in  injury  to  the  trees 
is  the  use  of  neat's  foot  oil.  Considerable  evidence  has  been  collected 
to  the  effect  that  this  substance  has  often  killed  considerable  bark  or 
furnished  a  condition  which  was  favorable  to  the  growth  of  the  para- 
sites. Experiments  in  the  laboratory  showed  that  Pythiacystis  citroph- 
thora  grew  as  well  on  the  surface  of  bark  previously  treated  with 
neat's  foot  oil,  as  on  that  which  had  not  been  treated,  and  Botrytis 
cinerea  grew  much  better  on  bark  to  which  neat's  foot  oil  had  been 
applied. 

Light,  penetrating  oils,  such  as  kerosene,  should  be  used  with  much 
caution  on  citrus  bark.  A  number  of  cases  have  been  observed  where 
kerosene  applied  at  the  base  of  the  tree  trunk  caused  the  death  of  the 
bark  at  and  below  the  surface  of  the  soil. 


MAL  DI  GOMMA,  OR  FOOTROT 

Mai  di  gomma,  due  to  Phytophthora  terrestria  Sherbakoff,  is  a 
gum  disease  with  close  relationships  to  Pythiacystis  gummosis.  Certain 
phases  of  the  Pythiacystis  gummosis  occurring  on,  or  near,  the  main 
roots  of  sweet-orange  trees,  are  quite  similar  to  those  of  mal  di  gomma, 
or  foot  rot.  For  this  reason,  certain  foot  rot-like  forms,  due  to  Pythia- 
cystis citrophthora  in  California,  have  previously  been  referred  to  as 
mal  di  gomma.  ^^'  ^"^  Since  the  term  mal  di  gomma  was  used  first  in 
Florida  to  designate  a  common  Florida  gum  disease,  which  is  now 
known  to  be  induced  by  Phytophthora  terrestria,  it  is  proposed  to 
restrict  its  use  (in  this  country  at  least)  to  the  disease  due  to  this 
fungus. 

SYMPTOMS 

This  type  of  gum  disease  affects,  for  the  most  part,  the  -bark  on 
the  lowest  portion  of  the  trunk  and  the  upper  portion  of  the  highest 
main  roots,  mostly  below  the  surface  of  the  soil.  Gum  usually  forms 
on  the  trunk  of  the  tree  above  the  soil.  The  inner  bark  and  finally 
the  wood  underneath  frequently  develop  a  fetid  odor.*     The  bark 

33  Smith,  K.  E.,  and  Butler,  O.,  Gum  Disease  of  Citrus  Trees  in  California. 
Calif.  Agr.  Exp.  Sta.  Bull.  200,  pp.  235-272,  1908. 

34  Fawcett,  H.  S.,  The  known  distribution  of  Pythiacystis  citrophtJiora,  and  its 
probable  relation  to  mal  di  gomma.    Phytopathology,  vol.  5,  pp.  66-67,  1915. 

*  This  rotting  of  the  wood,  as  well  as  the  bark,  and  the  accompanying  fetid 
odor  are  believed  to  be  due  mainly  to  secondary  organisms  setting  up  fermenta- 
tion and  decay  below  the  surface  of  the  soil  after  killing  of  the  bark  by 
the  primary  organism.  While  gum  may  be  formed  below  as  well  as  above  the 
surface  of  the  soil,  it  is  dissolved  readily  by  moisture  and  is  usually  less  con- 
spicuous below  the  soil  surface.  This  disease,  under  California  conditions, 
cannot  be  distinguished  from  certain  phases  of  brown  rot  gummosis  except  by 
means  of  culture  tests  for  isolating  the  causal  organisms. 


398  UNIVERSITY    OF    CALIFORNIA — EXPERIMENT   STATION 

dies  and  breaks  away  in  patches,  leaving  bare,  dead  areas,  wbicli  spread 
in  all  directions,  but  mostly  downv^ard,  on  the  main  crov^n  roots  and 
laterally  around  the  trunk  (fig.  8).  Trees  thus  affected  bear  heavy 
crops  of  fruit  temporarily  and  the  leaves  become  yellovr. 

HISTOEY 

This  gum  disease  was  first  known  as  foot  rot  and  attracted  attention 
in  Florida  about  the  same  time  that  Pythiacystis  gummosis  was  noticed 
in  California.  Curtiss,-^*'^  in  1888,  reported  that  it  appeared  in  1876, 
although  few  people,  he  says,  remember  having  observed  it  before 
1880.  That  the  disease  was  not  important  in  Florida  before  that  time 
is  indicated  by  the  fact  that  Bishop, ^^  writing  on  citrus  culture  in 
Florida  in  1875,  discusses  a  number  of  other  diseases  but  does  not 
mention  this  one. 

In  a  book  published  in  1881,  Moore,^^  who  made  extensive  observa- 
tions on  citrus  culture  in  Florida,  speaks  of  this  disease  under  ''foot 
rot*'  as  having  appeared  in  "late  years"  in  Florida.  In  1896  Swingle 
and  Webber^^  stated  that  the  disease  was  widely  distributed  in  Florida 
and  seemed  to  be  gradually  spreading. 

INVESTIGATIONS 

Only  once  has  the  causal  fungus  been  isolated  in  California.  This 
was  from  an  orange  tree  at  Lindsay  in  1912.  It  was  considered  by 
the  writer  at  that  time  to  be  only  a  peculiar  strain  of  the  brown  rot 
fungus,  Pythiacystis  citrophthora,  but  was  later  identified  by  Sherba- 
koff  as  the  same  species  which  he  had  previously  described.^^  This 
species,  or  a  closely  allied  one,  appears  to  be  widely  distributed,  occur- 
ring in  a  number  of  countries.  It  was  found  on  citrus*^'  *^  in  Florida, 
Cuba  and  Argentine,  on  coconuts,  tobacco  and  pineapples  in  Ja- 
maica,*^ on  tomatoes  in  Florida^^  and  castor  oil  plants  and  Vinca  in 
India.^^ 


35  Curtiss,  Sore  Shin  or  Gum  Disease.     Fla.  Agr.  Exp.  Sta.  Bull.  2,  1888. 

36  Bishop,  P.  P.,  Proc.  Am.  Porno.  Soc,  p.  48,  1875. 

37  Moore,  T.  W.,  Treaties  and  handbook  of  orange  culture,  New  Yor^,  1881. 

38  Swingle,  W.  T.,  and  Webber,  H.  J.,  The  principal  diseases  of  citrus  fruits 
in  Florida.     U.  S.  D.  A.,  Div.  of  Veg.  Phys.  and  Path.  Bull.  8,  1896. 

39  Sherbakoff,  C.  D.,  Buckeye  rot  of  tomato  fruit.  Phytopathology,  vol.  7, 
pp.  119-129,  1917. 

40  Fawcett,  H.  S.,  Pythiacystis  and  Phytophthora.  Phytopathology,  vol.  10, 
pp.  397-399,  1920. 

41  Stevens,  H.  E.,  Florida  citrus  diseases.  Fla.  Agr.  Exp.  Sta.  Bull.  150, 
110  pp.,  1918. 

42  Ashby,  S.  F.,  Leaf-stalk  rot  caused  by  Phytophthora  parasitica.  In  west 
Indian  Bull.  vol.  18,  pp.  70-73,  1920. 

43  Dastur,  J.  F.,  Phytophthora  parasitica  new  species;  a  new  disease  of  the 
castor  oil  plant.  Mem.  Dept.  Agr.  India,  Bot.  Ser.,  vol.  5,  pp.  177-231,  1913; 
idem.,  Phytophthora  on  Vinca  rosea.  Mem.  Dept.  Agr,  India,  Bot.  Ser.,  vol.  8, 
pp.  233-242,  1916. 


Bulletin  360]     gUM  DISEASES  OF  CITRUS  TREES  IN  CALIFORNIA 


399 


Fig.  7. — Mai  di  gomma  or  foot  rot.  (a)  On  40-year-old  seedling  orange  trees. 
White  lines  indicate  boundaries  of  killed  bark,  (h)  Earth  dug  away  and 
effected  bark  and  roots  being  removed  before  applying  Bordeaux  paste. 


400  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

Many  comparative  inoculations,  with  Pythiacystis  citrophthora, 
the  cause  of  brown  rot  gummosis,  and  Phytophthora  terrestria,  the 
cause  of  mal  di  gomma,  were  made  under  various  conditions  and  the 
same  type  of  lesion  was  produced  by  both  fungi.  There  was  an  indi- 
cation in  some  of  the  experiments  that  sweet-orange  bark  was  more 
susceptible  than  lemon  to  Phytophthora  terrestria  while  the  reverse 
was  generally  true  of  the  fungus  of  brown-rot  gummosis.  According 
to  Hume^*  in  Florida  the  sweet-orange  is  more  susceptible  to  mal  di 
gomma  than  the  common  lemon,  and  sour-orange  is  very  resistant. 

The  relation  of  temperature  to  growth  is  somewhat  different  for 
the  two  fungi. *^  While  the  optimum  temperature  for  sustained  growth 
over  a  period  of  several  days  in  the  laboratory  for  Pythiacystis  citroph- 
thora  is  about  77°F.  (25°C.),  that  for  Phytophthora  terrestria  is 
about  86°F.  (30°C.).  It  is  of  interest  in  this  connection  that  the 
rainy  season  in  California  occurs  during  winter  and  spring,  coinci- 
dent with  moderate  temperatures,  while  the  moist  season  in  Florida 
occurs  during  the  summer,  coincident  with  much  higher  temperature. 


CONTEOL 

The  means  of  prevention  and  treatment  of  mal  di  gomma,  are 
essentially  the  same  as  those  for  brown-rot  gummosis,  a  certain  form  of 
which  it  so  closely  resembles  that  only  laboratory  examination  can 
distinguish  the  two  diseases.  Care  regarding  excessive  moisture  in 
contact  with  the  bark  of  the  crown  roots  and  the  base  of  the  trunk, 
care  in  keeping  the  soil  away  from  the  base  of  the  tree  and  in  avoiding 
injuries,  the  use  of  sour-orange  stocks  for  new  plantings,  the  use  of 
fungicidal  washes  on  the  bark,  are  all  useful  in  prevention.  For 
detailed  suggestions  regarding  prevention  and  treatment,  *  see  under 
brown-rot  gummosis. 


44  Hume,   H.   H.,   Some   Citrus  troubles.     Fla.   Agr.  Exp.   Sta.  Bull.   53,  pp. 
145-173,  1900. 

45  Fawcett,  H.  S.,  The  temperature  relations  of  growth  in  certain  parasitic 
fungi.     Univ.  Calif.  Publ.  Agr.  Sci.,  vol.  4,  pp.  183-232,  1921. 


Bulletin  360]     qUM  DISEASES  OF  CITRQS  TREES  IN  CALIFORNIA  401 


BOTRYTIS  GUMMOSIS 

SYMPTOMS  AND  OCCUEEENCE 

Botrytis  gummosis*^  differs  from  brown-rot  (Pythiacystis)  gum- 
mosis,  in  that  it  causes  softening  of  the  invaded  bark  in  the  early 
stages  and  shows  a  grey  color  on  the  surface  in  damp,  cool  weather, 
caused  by  the  conidiophores  and  spores  of  the  fungus  (fig.  9).  In 
the  later  stages  the  outer  layer  of  bark  is  killed  and  becomes  dry  and 
hard  much  in  advance  of  the  inner  layer,  while  there  is  a  greater 
tendency  than  in  brown-rot  gummosis  for  the  tree  to  renew  the  bark 
underneath  the  dead,  hard  layer,  and  there  is  usually  also  a  less 
copious  flow  of  gum.  Unlike  brown-rot  gummosis,  Botrytis  gummosis 
is  confined  in  California  almost  exclusively  to  lemon  trees  growing 
in  the  coastal  regions,  and  usually  occurs  on  trees  that  are  more  than 
10  years  of  age.  This  disease  should  not  be  confused  with  ''shell 
bark, ' '  a  desquamated  bark  condition  in  which  the  outer  bark  of  lemon 
trees  dies,  cracks  and  breaks  away  in  longitudinal  strips,  a  condition 
which  is  somewhat  similar  to  that  frequently  brought  about  in  the 
later  stages  of  Botrytis  gummosis.  These  two  diseases  are  often  asso- 
ciated on  the  same  trees.  The  conditions  favorable  to  the  one  are 
also  apt  to  encourage  the  other.  Neither  disease  should  be  confused 
with  psorosis  (scaly  bark)  of  sweet-orange  trees. 

Unlike  Pythiacystis  citrophthora,  this  fungus  is  not  able  to  gain 
entrance  except  through  some  wound  or  defect  in  the  bark,  and  is 
not  able  to  progress  so  rapidly  in  killing  the  bark  through  to  the  wood. 
A  large  area  is  involved,  in  which  only  certain  outer  layers  of  bark 
tissue  are  killed,  leaving  the  cambium  alive  and  capable  of  renewal. 
This  gummosis  produces  an  outer  gummous  zone  beyond  the  invaded 
area,  but  this  is  usually  less  extensive  and  less  rapidly  formed  than 
with  Pythiacystis  gummosis.  Other  conditions  being  equal,  there  is 
usually  somewhat  less  gum  formation  in  Botrytis  gummosis  than  in 
Pythiacystis  gummosis. 

The  writer's  attention  was  first  called  to  this  type  of  gummosis 
early  in  February,  1912.  After  a  period  of  moist,  cool  weather,  patches 
of  bark  6  to  12  inches  long  and  half  as  wide  presented  the  gray  furry 
appearance  characteristic  of  the  fruiting  bodies  of  Botrytis  cinerea 
(fig.  8).  In  a  later  survey  of  th-e  citrus  districts  of  California,  Botrytis 
cinerea  was  always  found  associated  with  this  type  of  gummosis  and 
was  isolated  from  a  large  number  of  diseased  trees. 


46  Fawcett,  H.  S.,  "Two  fungi  as  causal  agents  in  gummosis  of  lemon  trees 
in  California,"  in  Month.  Bull.  Calif.  Comm.  Hort.,  vol.  2,  pp.  601-617,  1913, 
and  Phytopathology,  vol.  4,  p.  54,  1914. 


402  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


INVESTIGATIONS  AS  TO  NATURE  AND  CAUSE 

The  investigations  establishing  the  relation  of  the  fungus  Botrytis 
cinerea  to  this  disease,  were  conducted  in  much  the  same  way  as  those 
in  connection  with  brown-rot  gummosis. 

(1)  Inoculation  with  diseased  tissue  showed  that  the  disease  could 
be  transmitted  to  sound  bark  of  healthy  trees. 

(2)  Cultures  from  the  diseased  lesions  showed  the  presence  of  a 
fungus,  Botrytis  cinerea,  previously  known  as  the  Botrytis  rot  fungus 
of  the  packing  houses. 

(3)  By  inoculation  experiments  with  pure  cultures  of  this  fungus 
the  characteristic  symptoms  were  induced  on  healthy  trees  (fig.  9). 

(4)  Inoculation  wdth  Botrytis  fungus  isolated  from  lemon  fruits 
also  produced  the  same  results. 

(5)  The  Botrytis  fungus  was  again  isolated  from  the  artificially 
induced  lesions  and  was  found  capable  of  inducing  rot  in  lemon  fruits 
irrespective  of  whether  it  had  been  found  originally  in  diseased  bark 
or  in  rotting  fruit.  Some  of  the  details  of  this  work  are  being  published 
in  the  Journal  of  Agricultural  Research. 

The  fungus  was  isolated  from  the  softened  invaded  area  of  a  large 
number  of  these  lesions.  Attempts  to  isolate  the  fungus  from  the 
outer  gummous  zone,  however,  failed,  just  as  they  did  in  Pythiacystis 
gummosis.  Only  rarely  was  Botrytis  isolated  from  the  area  where 
the  outer  bark  was  dead  and  hard.  Cultures  showed  that  following 
Botrytis  inoculation  this  outer  dead  cortical  layer  is  rapidly  occupied 
under  such  conditions  as  prevail  at  Santa  Paula  by  species  of  Alter- 
naria,  Cladosporium,  Penicillium,  Colletotrichum,  Fusarium,  and  other 
fungi  and  bacteria. 

FACTORS  FAVORING  THE  DISEASE 

Many  contributing  conditions  tend  to  favor  the  occurrence  and  the 
severity  of  this  disease.  Some  of  these  are  similar  to  those  which 
favor  Pythiacystis  gummosis,  as  discussed  under  that  disease. 

Injuries  of  various  kinds  to  the  bark,  not  only  near  the  soil,  but 
anywhere  on  the  trunk  or  large  branches,  may  lead  the  way  to  infec- 
tion and  development  of  Botrytis  gummosis  when  the  conditions  of 
moisture  and  temperature  are  also  favorable.  This  disease  is  frequently 
severe  on  living  tissue  of  trees  that  have  been  injured  by  frost.  The 
fungus  may  first  become  established  in  such  trees  in  a  small  portion 
of  dead  or  dying  tissue  and  then  advance  rapidly  into  tissue  which 
appears  to  be  sound. 


Bulletin  360]     oi^r  DISEASES  OF  CITRUS  TREES  IN  CALIFORNIA 


403 


Fig.  8. — Grey  masses  of  spores  of  Botrytis  cinerea  on  the  surface  of  bark 
of    a    Botrytis    gummosis    lesion. 


404  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

A  desquamated  condition  of  bark,  fairly  common  on  old  lemon 
trees  in  the  California  coastal  regions,  is  also  frequently  accompanied 
by  Botrytis  gummosis.  It  furnishes  dead  outer  bark  tissue  from 
which  the  fungus  may  advance.  The  desquamated  condition  is  similar 
in  appearance  to  that  which  usually  follows  inoculation  with  Botrytis 
cinerea  on  sound  tree  trunks  and  Avith  which  it  is  often  confused.  It  is 
thought,  however,  to  be  due  to  other  causes.  Recent  experiments  have 
shown  that  a  species  of  Phomopsis,  to  be  described  as  P.  calif ornica  is 
probably  a  factor  in  causing  "shell  bark." 

The  previous  use  of  neat's  foot  oil  in  the  treatment  of  gummosis 
encouraged  the  growth  of  the  Botrytis  fungus.  The  trunks  of  lemon 
trees  previous^  treated  at  Santa  Paula  by  scoring  the  bark  and 
painting  with  neat's  foot  oil  were  observed  in  February  and  March 
of  1912  to  be  fairly  well  covered  with  a  gray  coating  consisting  of  the 
sporophores  and  spores  of  Botrytis.  The  bark  on  these  trees  was 
found  to  be  in  various  stages  of  soft  decay  with  the  exudation  of 
large  masses  of  gum.  Experiments  also  showed  that  this  fungus 
develops  better  on  lemon  bark  treated  with  neat 's  foot  oil  either  before 
of  after  infection  by  the  organism  than  on  bark  free  from  this  oil. 
More  recently  the  application  of  neat's  foot  oil  to  citrus  trees  has 
been  largely  given  up,  and  the  more  severe  stages  of  this  disease,  such 
as  were  previously  seen,  have  not  been  observed  lately. 

METHODS    OF    CONTROL 

The  control  methods  used  for  Botrytis  gummosis  as  in  the  case 
of  Pythiacystis  gummosis  are  of  two  kinds,  prevention  and  treatment ; 
both  of  which  are  similar  in  principle  to  those  discussed  in  connection 
with  the  former  disease.  A  few  modifications,  however,  based  on  the 
differences  in  the  latter  disease  should  be  pointed  out. 

Prevention.— Since  Botrytis  cinerea  appears  to  be  dependent  upon 
abrasions  or  other  injuries  for  its  entrance  into  lemon  bark,  especial 
care  is  necessary,  particularly  in  moist  weather,  to  avoid  injuries  in 
cultivation,  picking  and  other  orchard  operations.  The  danger  from 
such  injuries  may  be  lessened  by  painting  the  tree  trunks  with  Bor- 
deaux paste  or  other  fungicides  or  spraying  them  thoroughly  with 
Bordeaux  mixture.  The  precaution  previously  mentioned  of  pulling 
away  soil  that  is  too  high  against  the  trunk  and  of  keeping  water  as 
much  as  possible  away  from  the  trunk,  are  also  applicable  in  the  case 
of  Botrytis  gummosis. 

Treatment. — The  principle  governing  the  treatment  of  this  type 
of  gum  disease  is  the  same  as  that  for  Pythiacystis  gummosis,  namely, 
the  elimination  of  the  invaded  tissue  and  the  prevention  of  further 


Bulletin  360]     gUM  DISEASES  OP  CITRUS  TREES  IN  CALIFORNIA 


405 


progress  of  the  disease.  As  the  result  of  many  different  experiments 
in  which  growers  took  a  prominent  part,  a  method  consisting  largely 
of  scraping  off  the  outermost  layers  of  bark  was  found  which  proved 
to  be  best  adapted  for  treatment  of  this  disease  (fig.  10),  The  portion 
where  the  bark  is  totally  killed  is  cut  away,  but  beyond  this  where 
usually  only  the  outer  layers  of  bark  are  dead,  these  outer  layers  only 
are  scraped  off,  leaving  intact  the  live  inner  layer  next  to  the  cambium. 


T'' 


f! 


A 


B 


Fig.  9, — (a)  Gum  exudation  on  a  lemon  trunk  as  the  result  of  inoculation 
with  the  Botrytis  fungus. 

(h)  Bark  over  and  around  the  diseased  lesion  scraped,  to  show  the  extent 
of  the  dead  soft  bark. 


To  prevent  further  invasion  of  the  outer  bark  it  is  usually  found  neces- 
sary to  scrape  the  sound  bark  several  inches  beyond  the  margin  of  the 
affected  region.  A  sharp  curved  tool,  described  under  "psorosis, " 
which  can  be  easily  controlled  in  scraping,  is  in  general  use  for  this 
purpose  (fig.  13).  This  modification  of  the  method  used  for  Pythia- 
cystis  gummosis  is  advisable  because  of  the  different  nature  of  the 
disease,  in  order  to  preserve  as  much  as  possible  of  the  inner  live 
bark.  Where  both  types  of  gummosis  are  present  on  the  same  trees, 
as  is  frequently  the  case,  this  method  is  still  applicable  to  the  com- 
bined lesions  produced.    The  cut  or  scraped  portions  are  then  painted 


406  UNIVERSITY    OF    CALIFORNIA — EXPERIMENT   STATION 

with  a  fungicide  (fig.  lOh).  Bordeaux  paste,  and  some  of  the  coal  tar 
products,  such  as  Avenarius  Carbolineum,  Arrow  Carbolineum  and 
Creolineum  XXX,  which  contain  only  the  heavier  oils,  have  given 
good  results. 

The  following  is  a  typical  example  of  the  many  experiments  in 
treatment  of  this  disease : 

On  October  8,  1912,  at  Santa  Paula,  a  lesion  on  the  trunk  of  an 
18-year-old  lemon  tree  was  treated  by  cutting  away  the  bark  entirely 
in  a  few  small  places  where  it  was  killed  to  the  wood,  but  scraping 
away  only  the  dead  outer  layers  and  leaving  the  cambium  still  attached 
(fig.  10a).  The  scraping  was  done  with  a  sharp  curved  instrument 
made  on  the  principle  of  a  box  scraper,  and  the  scraped  portion  was 
then  painted  with  Bordeaux  paste  (fig.  10&). 

On  May  10,  1913,  new  bark  was  building  up  over  the  scraped 
portion,  but  the  disease  had  spread  slightly  on  the  margins  where 
scraping  had  not  been  continued  out  far  enough. 

On  May  17,  1914,  new  bark  was  seen  to  have  formed  over  the 
entire  scraped  portion  and  the  disease  appeared  to  be  entirely  stopped. 

SCLEROTINTA   GUMMING  DUE   TO   SCLEROTINIA 

LIBEKTIANA 

This  disease,  usually  of  minor  importance,  occasionally  has  been 
found  associated  with  rapid  dying  of  bark  on  the  roots  and  trunks  of 
citrus  trees  growing  in  damp,  cool  locations  especially  after  periods  of 
severe  frost.  The  bark  is  at  first  soft  just  as  in  the  case  of  attack  by 
Botrytis  cinerea.  Though  this  fungus  usually  advances  more  rapidly 
than  Botrytis,  it  is  soon  checked,  and  callus  begins  to  form  as  soon  as 
gum  accumulates.  Later,  as  the  bark  dries,  it  is  left  in  shreds  (fig.  11) 
and  large  black  sclerotia  are  found  within  and  under  this  bark.  Its 
effect  on  citrus  twigs  has  been  described  by  C.  0.  Smith, *^  who  refers 
to  the  gumming  usually  accompanying  its  attack.  It  appears  to 
infect  the  young  growth,  usually  at  the  blossoming  period,  and  fre- 
quently extends  back  into  larger  branches. 

When  the  fungus  is  found  on  the  trunk  or  roots,  observations  have 
indicated  that  previous  injury  of  the  bark  was  usually  necessary  for 
its  entrance.  It  has  frequently  been  found  on  young  trees  following 
frost  injuries,  apparently  advancing  from  frost  injured  tissue  into 
tissue  not  killed  by  frost.  It  has  been  observed  on  a  lemon  tree  20 
years  old  where  all  the  roots  had  been  infected,  probably  from  injuries 
made  in  digging  about  them  and  placing  vetch  straw  near  the  crown 


47  Smith,  C.  O.,  Cottony  rot  of  lemons  in  California.     Calif.  Agr.  Exp.  Sta. 
Bull.  265,  pp.  237-258,  1916. 


Bulletin  360]     gxJM  DISEASES  OF  CITP.US  TREES  IN  CALIFORNIA 


407 


in  damp  cool  weather.  An  old  seedling  orange  tree  also  was  observed 
with  the  bark  on  one  side  of  the  trunk  killed  by  the  fungus,  which 
had  apparently  gained  entrance  through  a  small  sunburned  area  and 
had  advanced  into  the  live  bark  for  some  distance. 


Fig.  10. — Treatment  of  Botn'tis  gummosis. 

(a)   Lesion  being  scraped. 

(&)   Painting  scraped  area  with  Bordeaux  paste. 

This  fungus  has  also  been  shown  by  inoculation  experiments  to  be 
capable  of  advancing  rapidly  into  lemon  bark,  inducing  copious  gum 
formation  for  a  short  time  only.  Observation  shows  that  the  halting 
of  the  invasion  of  the  fungus  is  usually  coincident  with  the  exudation 
of  considerable  quantities  of  gum.  The  prevention  and  treatment  of 
this  disease  is  the  same  as  that  for  Botrytis  gummosis.  (See  directions 
under  Botrytis  gummosis,  page  404.) 


408  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 


PSOROSIS  (SCALY  BARK)  OF  ORANGE  TREES 

Althoiigh  psorosis  is  generally  classed  as  a  gum  disease,  gum 
exudes  only  at  certain  seasons,  or  during  certain  phases  of  develop- 
ment of  the  disease,  and  then  the  gum  is  usually  not  so  conspicuous  a 
feature  as  it  is  with  brown-rot  gummosis. 

This  disease  was  briefly  described  by  W.  T.  Swingle  and  H.  J. 
Webber  in  1896  in  Florida  and  given  the  name  of  psorosis. 

It  is  now  known  in  California  as  ''scaly  bark,"  but  must  not  be 
confused  with  another  disease  known  in  Florida  as  scaly  bark  (nail- 
head  rust).  This  last  is  distinct  from  psorosis  and  does  not  occur  in 
California. 

The  slowness  of  the  development  of  the  disease,  and  its  inconspicu- 
ous appearance  and  lack  of  effect  upon  the  foliage  in  the  earlier  stages, 
frequently  prevent  it  from  being  noticed  until  it  is  far  advanced. 

SYMPTOMS 

The  most  conspicuous  feature  of  the  scaly-bark  disease  is  the 
appearance  on  the  trunk  or  large  limbs,  of  irregular  scales  of  bark 
%  to  1  inch  in  diameter,  standing  out  as  if  pushed  off  from  the 
surface  (fig.  12).  It  usually  begins  with  a  very  small  area  in  which 
only  a  thin  outer  layer  of  bark  dies,  hardens,  and  is  raised  from  the 
surface,  leaving  a  layer  of  bark  underneath  still  alive.  This  first  area 
slowly  enlarges  from  year  to  year  until  finally  it  encircles  the  trunk 
or  limb.  Often  a  number  of  small  areas  begin  at  the  same  time  and 
thus  cover  the  surface  more  rapidly.  Later  the  deeper  layers  of  bark 
and  even  the  wood  may  be  affected.  Cum  may  exude  as  the  disease 
advances,  but  gum  is  not  a  necessary  accompaniment  of  the  disease 
and  its  presence  and  amount  depend  on  growth  conditions,  season  of 
the  year,  etc. 

INVESTIGATIONS    AS    TO    CAUSE    AND    MANNEE    OF    DEVELOPMENT 

An  investigation  into  the  cause  and  manner  of  development  and 
control  of  this  disease,  has  been  carried  on  for  years.  Many  difficulties 
which  were  not  encountered  in  the  work  with  brown-rot  gummosis 
have  arisen  in  the  investigation  of  the  cause  of  psorosis.  One  of  these 
has  been  the  extreme  slowness  with  which  the  disease  develops  in  its 
incipient  stages. 

In  certain  cases  it  has  been  possible  to  transmit  the  disease  to  sound 
trees  by  inoculations  with  bits  of  tissue  from  diseased  lesions,  but 
many  of  the  attempts  have  failed.  In  one  successful  case  two  years 
elapsed  after  the  diseased  tissue  was  placed  in  a  wound  in  sound  bark. 


Bulletin  360]     quM  DISEASES  OF  CITRUS  TREES  IN  CALIFORNIA 


409 


before  any  sign  of  the  disease  was  noted.  This  suggests  that  an 
organism  of  some  kind  which  is  able  to  advance  very  slowly  may  be 
the  immediate  cause  of  the  disease.    Experiments  planned  to  determine 


Fig  11. — Eoot  of  lemon  tree  showing  shredded  appearance  of  the  bark  and 
the  black  sclerotia  as  the  result  of  the  attack  of  Sclerotinia  libertiani  .  Arrows 
point  to  sclerotia. 


whether  this  hypothesis  is  correct  have  been  under  way  for  some  time, 
but  because  of  the  slowness  of  development  of  Psorosis  from  the  early 
stages  there  has  not  been  time  for  proof  of  this  conjecture  to  be 
established. 


410  UNIVERSITY    OF    CALIFORNIA — EXPERIMENT    STATION 

As  examples  of  the  slowness  with  which  the  disease  often  develops, 
the  following  cases  may  be  mentioned.  At  the  Experiment  Station 
two  incipient  lesions  not  yet  gumming-,  and  each  about  1  inch  in 
diameter  were  outlined  on  a  Valencia  orange  tree  12  years  old.  These 
lesions  had  progressed  only  Y^  inch  beyond  the  original  mark  in  21/2 
years.  A  slightly  larger  area  on  another  tree,  which  was  1%  inches  in 
diameter  when  first  observed  became  only  4  inches  in  diameter  in  3I/2 
years.  These  areas  were  so  small  and  inconspicuous  when  outlined  that 
they  would  not  have  been  noted  at  all  at  that  stage  by  the  average 
grower. 

After  the  lesions  become  larger,  however,  they  develop  somewhat 
more  rapidly.  The  following  example  represents  a  rather  rapidly 
developing  lesion.  An  area  on  a  trunk  of  a  12-year-old  Valencia  tree 
was  3x7  inches  when  marked.  Six  months  later  it  was  8x12  inches; 
after  9  months,  10x15  inches;  after  2  years,  12x16  inches  (with  much 
gumming),  and  after  3I/2  years  14x17  inches.  While  this  increase 
from  3x7  inches  to  14x17  inches  seems  considerable  in  itself,  it  repre- 
sents an  average  of  only  about  li/^  inches  of  advance  per  year  in  any 
one  direction.  The  lesion  was  probably  3  to  4  years  old  when  first 
measured.  Scaly  bark  thus  differs  from  any  other  gum  diseases, 
whose  whole  course  of  development  can  be  followed  out  in  1  or  2 
years. 

The  visible  advance  of  the  disease  depends  greatly  upon  the  season 
of  the  year.  In  the  majority  of  cases  it  seems  to  be  most  active  during 
the  growing  season  and  especially  in  summer  and  early  fall,  and  to  be 
quiescent  during  the  winter  and  early  spring.  This  great  show  of 
activity  is  due  in  part  to  the  breaking  away  of  scales  of  bark,  probably 
brought  about  not  only  by  drying  of  the  outer  bark  that  has  previously 
died,  but  also  by  the  increase  in  growth  of  the  inner  live  bark  beneath. 
Gum  formation  and  exudation  also  take  place  at  this  season,  giving 
further  indication  of  activity.  Most  of  the  advance  for  the  year, 
however,  appears  to  be  finished  when  the  gum  formation  is  at  its 
maximum.  The  gum  is  the  result  and  not  the  cause  of  the  death  of 
the  outer  bark.  The  gum  probably  tends  to  hinder  more  than  to  aid 
the  advance  of  the  disease  at  this  time. 

EXPERIMENTS  IN  TREATMENT 
Although  the  investigation  as  to  the  cause  and  manner  of  develop- 
ment of  the  disease  received  first  attention,  a  number  of  experiments 
in  control  have  also  been  carried  on.  Various  methods  of  cutting, 
scraping  and  slitting  the  bark  have  been  compared  with  no  cutting  or 
scraping.  Comparisons  have  also  been  made  of  the  application  of  a 
considerable  number  of  substances  as  fungicides  or  coverings. 


Bulletin  360]     gUM  DISEASES  OF  CITRUS  TREES  IN  CALIFORNIA 


411 


Without  going  into  detail,  it  may  be  said  that  a  method  of  scrap- 
ing the  outer  bark  to  depths  varying  according  to  circumstances,  to  be 
described  later,  has  been  found  more  effective  than  any  other  method. 
Scraping  has  been  found  to  be  much  more  important  than  the  use  of 
any  kind  of  fungicide  or  disinfectant.  Slitting  has  proven  of  little 
or  no  value  in  these  experiments.  These  experiments  have  also  shown 
the  advisability  of  scraping  not  only  the  area  visibly  affected,  but  also 
beyond  the  area  to  a  distance  of  from  6  to  8  inches  to  head  off  the 
advance  of  the  disease  in  bark  not  yet  visibly  affected.  The  spread  of  a 
lesion  in  its  early  stages  appears  to  take  place  within  the  outermost 
layers  of  bark.  This  accounts  for  the  beneficial  results  of  light  scrap- 
ing beyond  the  visible  edge  of  the  lesions. 

A  large  number  of  disinfectants  and  coverings  were  tested  with 
and  without  scraping  or  other  method  of  cutting  the  bark.  In  each 
case  similar  areas  on  the  same  or  different  trees  were  left  without  the 
application  of  the  disinfectant  for  comparison.  Almost  without  excep- 
tion the  diseased  trees  on  which  the  bark  was  carefully  scraped  showed 
the  best  recovery.  The  disinfectant  applied  seemed  to  have  little 
effect.  Lesions  of  the  same  kind  not  scraped  continued  to  develop  in 
size  at  the  same  rate,  whether  painted  or  unpainted  with  the  various 
disinfectants. 

A  cooperative  experiment  was  also  carefully  conducted  under  com- 
mercial conditions  by  M.  B.  Rounds*^  of  the  Azusa  Foothill  Citrus 
Company.  Methods  of  scraping  and  slitting  the  bark,  combined  with 
applications  of  Bordeaux  paste,  Creolineum  XXX  and  emulsified 
cresol  (liquor  cresolis  compositus  U.  S.  P.),  respectively,  with  check 
trees  for  comparison,  were  instituted  according  to  the  following  plan : 
Most  of  the  trees  treated  were  in  the  first  and  second  stages  and  were 
chosen  with  an  attempt  to  get  sets  of  like  specimens  for  each  combina- 
tion of  treatment.  . 


Check 

Bordeaux 

Creolineum 

Emulsified 
cresol 

Check,  not  scraped 

4  trees 

5  trees 
5  trees 
5  trees 

5  trees 
5  trees 
5  trees 
5  trees 

5  trees 
5  trees 
5  trees 
5  trees 

5  trees 

Scraped 

6  trees 

Scraped  and  slit 

5  trees 

Slit 

5  trees 

The  scraping  was  done  according  to  the  method  described  below 
and  the  slitting  by  making  long  vertical  slits  with  a  heavy  knife, 
through  and  on  either  side  of  the  diseased  lesions.     The  trees  were 

48  Eounds,  M.  B.,  California  Cultivator,  vol.  8,  p.  222,  1922. 


412 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT   STATION 


treated  in  October,  1921,  and  the  following  results  are  reported  by  Mr. 
Bounds  for  August,  1922. 


Disease  still 

progressing 

Number  of  trees 


Disease  not 

progressing 

Number  of  trees 


Not  scraped  or  slit. 


Check 

Bordeaux 

Creolineum  XXX. 
[  *Emulsified  cresoL. 


Scraped. 


f    Check 

J     Bordeaux 

]     Creolineum  XXX. 
Emulsified  cresol.. 


Scraped  and  slit. 


Check 

Bordeaux 

Creolineum  XXX. 
Emulsified  cresol.. 


Slit  only. 


Check 

Bordeaux 

Creolineum  XXX. 
Emulsified  cresol... 


3 
4 
5 
4 

16 

0 
0 
0 
0 

0 

0 
0 
2 
0 


3 
3 

4 
3 

13 


1 

1 
0 
1 


5 
5 
5 
6 

21 

5 
5 
3 
5 

18 

2 
2 
1 

2 


*  Liquor  creFolis  compositus  U.  S.  P.,  full  strength. 

While  it  is  too  soon  to  draw  definite  conclusions  from  this  experi- 
ment alone,  because  of  the  slowness  of  development  of  the  disease,  the 
results,  taken  together  with  experiments  made  at  the  Experiment 
Station  and  many  other  observations,  indicate  that  the  particular 
fungicide  to  be  applied  is  of  less  importance  than  the  manner  of  scrap- 
ing or  treating  the  bark.  None  of  the  trees  scraped  alone  showed  any- 
advance  of  the  disease,  while  65  per  cent  of  those  slit  alone,  and 
approximately  80  per  cent  of  those  not  scraped  or  slit,  showed  an 
an  advance  in  the  disease  at  the  time  here  recorded. 


Bulletin  360]     quM  DISEASES  OF  CITRUS  TREES  IN  CALIFORNIA 


413 


SUGGESTIONS  FOR  TREATMENT 

The  stage  of  the  disease  largely  determines  how  each  tree  should  be 
handled.  Each  tree  affected  presents  an  individual  problem.  Certain 
tentative  suggestions,  however,  will  be  made  here,  it  being  understood 
that  these  apply  to  typical  conditions  and  that  they  may  need  modifi- 
cation when  applied  to  any  particular  case. 


Fig,  12. — About  the  second  stage  of  psorosis  (scaly  bark)  on  orange  trees, 
showing  formation  of  scales  of  bark,  giving  the  surface  a  roughened  scaly 
appearance. 


First  Stage. — At  the  very  beginning  only  an  outer  layer  of  bark 
appears  to  be  injured  or  changed,  an  inner  layer  next  to  the  cambium 
still  being  alive  and  active  and  free  from  discoloration  except  that  it 


414  UNIVERSITY    OF    CALIFORNIA EXPERIMENT   STATION 

frequently  presents  a  slight  greenish  appearance.  Later  a  yellowish 
discoloration  may  appear,  due. to  the  development  of  gum  within  the 
tissue.  The  trunk  and  large  limbs  of  all  trees  should  be  inspected 
carefully  to  detect  the  disease  at  its  very  first  beginning.  When  these 
beginning  areas  are  small  and  do  not  cover  more  than  1/4  ^^  "the  circum- 
ference, the  affected  bark  may  be  scraped  rather  deeply  and  the  sur- 
rounding apparently  unaffected  bark  scraped  very  lightly  for  4  to  6 
inches  in  all  directions  beyond  the  margin  of  the  affected  areas. 

Second  Stage. — When  the  disease  has  progressed  further,  so  as  to 
cover  more  than  about  %  of  the  circumference  of  the  entire  bark  on 
the  trunk  but  has  not  yet  seriously  injured  the  wood,  it  may  for 
convenience  be  considered  to  be  in  the  second  stage.  At  this  stage 
the  affected  portion  often  presents  a  roughened  surface  (fig.  12). 

Fresh  scaling  of  the  bark  on  the  advancing  edges  continues  and 
exudation  of  gum  takes  place  at  certain  seasons  of  the  year.  In  such 
cases  the  cure  is  more  uncertain,  but  the  disease  may  often  be  checked 
and  sometimes  cured  by  a  light  to  medium  scraping.  Particular  atten- 
tion should  be  given  to  the  advancing  edges.  Care  must  be  taken  not 
to  scrape  deeply  enough  to  kill  the  inner  layer  of  bark.  The  affected 
surface  may  then  be  treated  with  a  disinfectant  as  previously  men- 
tioned. Six  months  or  a  year  later,  these  should  be  treated  again, 
scraping  only  where  the  disease  is  still  active.  The  progress  of  the 
disease  is  so  slow  that  usually  one  cannot  discern  within  less  than  six 
months  or  a  year,  whether  it  has  progressed  or  not. 

Third  Stage. — Where  the  disease  has  been  present  for  a  number 
of  years  (5  to  10  or  more)  or  until  a  greater  part  of  the  bark  of  the 
trunk  is  affected  and  the  wood  underneath  is  killed  and  beginning  to 
decay,  there  is  little  hope  for  a  permanent  recovery.  A  tree  of  this 
kind,  however,  sometimes  remains  surprisingly  productive  for  a  num- 
ber of  years,  so  that  it  becomes  a  question  whether  to  replace  it  at 
once  or  to  treat  it  superficially  with  the  idea  of  preventing  possible 
spread  to  other  trees,  and  of  taking  it  out  later.  If  only  a  part  of  the 
branches  show  the  disease  in  the  second  and  third  stages,  these  may 
be  cut  out  entirely  and  the  remaining  part  of  the  tree  frequently  in- 
spected for  further  outbreaks.  If  the  wood  is  just  beginning  to  be 
discolored  and  killed,  this  should  be  chisled  out  and  benzene-asphalt 
paint  or  other  good  wood  protecting  covering  applied  to  the  exposed 
surfaces.  A  severe  cutting  back  of  a  badly  affected  tree  may  be 
helpful.  If  the  trunk  is  too  badly  decayed  and  the  entire  tree  appears 
stunted  and  unproductive,  the  tree  should  be  dug  out  at  once. 

Season  of  the  Year. — Where  there  is  much  frost  hazard,  bark 
scraped  too  late  in  the  fall  or  during  the  winter  months  is  likely  to  be 


Bulletin  360]     qUM  DISEASES  OF  CITRUS  TREES  IN  CALIFORNIA 


415 


killed  by  low  temperatures.  Experiments  at  Riverside  indicate  that 
bark  treated  in  the  late  spring  and  summer  months  recovers  most 
rapidly. 

Fungicides. — As  was  previously  stated,  the  kind  of  fungicide 
employed  appeared  to  be  of  minor  importance  in  certain  of  our  experi- 
ments. Some  kind  should  be  applied,  how- 
ever, to  the  scraped  areas,  as  a  matter  of 
precaution.  Except  for  the  slight  danger 
to  the  foliage  when  followed  by  fumigation 
there  is  nothing  better  than  Bordeaux  paste. 
Other  substances  of  good  fungicidal  value 
are  some  of  the  high-boiling  coal  tar  prod- 
ucts such  as  Arrow  Carbolineum,  Creolin- 
eum  XXX,  etc.  Mercuric  cyanid,  1  part 
dissolved  in  500  parts  of  water  and  500 
parts  alcohol,  as  used  for  pear  blight,  is 
also  an  excellent  disinfectant.  There  are 
many  others  that  might  be  used. 

For  Bordeaux  paste,  dissolve  1  pound 
of  bluest  one  (copper  sulfate)  in  3  quarts  of 
water  in  a  wooden,  earthen  or  glass  vessel 
and  slake  2  pounds  of  lime  in  3  quarts  of 
water.  The  bluestone  is  most  easily  dis- 
solved by  suspending  it  in  a  sack  at  the  top 
of  the  water  overnight.  If  the  bluestone  is 
pulverized  and  suspended  in  warm  water 
it  dissolves  rapidly.  Good  lime  that  is  not 
air-slaked  should  be  used.  If  covered  to 
avoid  evaporation  the  dissolved  ingredients 
will  keep  indefinitely  in  separate  vessels. 
\Yhere  the  paste  is  being  used  over  a  num- 
ber of  days  or  weeks,  just  enough  of  the  wet 
slaked  lime  and  the  bluestone  solution 
should  be  mixed  to  last  for  one  or  two  days. 
It  may  be  applied  with  large  whitewash 
brushes.  Commercial  Bordeaux  pastes 
brought  to  equivalent  strength  may  also  be 
used. 

Tools. — A  number  of  different  kinds  of 

«  ■         .1      T      ^  Fiff.  13. — Tool  for  scrap- 

scrapers  are  in  use  lor  scraping  the  bark,     jj^g  ^gg  ^ark  or  for  goug- 

Several  types  devised  by  Mr.  Culbertson  of    i^g  o^^t  small  diseased  areas 

■  IT-  •         /-<  e  •       ji         caused  bv  psorosis  or  other 

the   Limoneira    Company   for   use   m   the     ^j^j.^  diseases. 


416  UNIVERSITY    OF    CALIFORNIA EXPERIMENT   STATION 

treatment  of  Botrytis  gummosis  are  shown  in  figure  10.  A  modifi.ca- 
tion  of  one  of  these,  first  made  under  the  direction  of  Dr.  J.  T.  Barrett 
of  the  Citrus  Experiment  Station,  is  now  in  common  use.  It  may  be 
made  of  spring  steel  and  consists  of  a  curved  blade  sharpened  on  both 
edges  and  at  the  end  and  set  in  a  wooden  handle.  The  handle  is 
714  inches  long  and  1%  inches  in  diameter  and  the  steel  part  con- 
taining the  curved  blade  is  5%  inches  long  (fig.  13).  These  may  be 
made  by  a  local  blacksmith  or  obtained  through  the  farm  advisor. 


DIPLODIA  GUMMING*^ 

In  a  former  publication^®  it  was  stated  that  this  form  of  gumming 
had  not  been  seen  in  California,  but  was  common  in  southern  Florida 
and  Cuba.  Since  then  a  Diplodia  fungus  similar  to  the  Florida  one 
has  frequently  been  found  in  California  especially  in  San  Diego  county 
associated  with  a  gumming  of  large  branches.  A  gummosis  due  to  a 
Diplodia  has  been  found  recently  in  the  Philippines  also.^^  This  fungus 
appears  to  be  especially  active  in  California  in  connection  with  the 
so-called  ''heart  rot"  following  severe  freezes.  After  the  freeze  of 
1913  this  fungus  was  found  advancing  into  the  yet  unkilled  wood  of 
large  branches  Avhich  had  been  cut  off  after  the  freeze,  resulting  in 
frequent  gum  formation.  The  fungus  advanced  much  more  rapidly 
in  branches  whose  cut  ends  were  sealed  with  grafting  wax  than  in 
those  not  so  sealed.  The  Diplodia  attack  resulted  indirectly  in  the 
death  of  areas  of  bark,  but  the  fungus  advanced  much  more  rapidly 
in  the  woody  tissue  (fig.  14). 

Control. — The  heart  rot  due  to  Diplodia  following  a  severe  freeze  is 
difficult  to  control.  A  non  air-tight  disinfectant,  such  as  Bordeaux 
paste,  mercuric  cyanid  (1  part  in  1000  parts  of  denatured  alcohol) 
or  other  non-injurious  substance,  should  be  used  to  disinfect  the 
tools  and  cut  surfaces.  This  treatment  may  be  followed  by  the  sealed 
covering  sometime  later  when  the  wound  has  thoroughly  dried  out. 
If  only  one  application  is  to  be  used,  a  thin  substance  like  the  higher 
boiling  coal  tar  products,  as  Avenarius  carbolineum,  Arrow  carbolin- 
eum,  or  Creolineum  XXX,  may  be  used.  All  parts  of  the  tree  cut 
back  should  be  thoroughly  whitewashed  to  prevent  sunburning. 

49  Fawcett,  H.  S.,  Diplodia  natalensis  as  a  gum-inducing  and  fruit-rotting 
fungus.  Report  of  Plant  Pathologist,  Fla.  Agr.  Exp.  Sta.  Ann.  Report,  1911, 
pp.  61-67,  1912. 

Gumming,  Report  of  the  former  Plant  Pathologist,  Fla.  Agr.  Exp.  Sta.  Ann. 
Report,  1912,  pp.  77-92,  1913. 

50  Fawcett,  H.  S.,  Citrus  diseases  of  Florida  and  Cuba  compared  with  those 
of  California.     Univ.  of  Calif.  Agr.  Exp.  Sta.  Bull.  262,  p.  210,  1915. 

81  Reinking,  O.  A.,  Philippine  Agriculturist,  vol.  9,  123-127,  1921. 


Bulletin  360]     quM  DISEASES  OF  CITRUS  TREES  IN  CALIFORNIA 


417 


TWIG  GUMMING 

A  gumming  and  dying  of  a  few  scattered  twigs,  especially  on  naval 
orange  trees,  in  the  late  summer  or  fall,  is  usually  of  minor  importance. 
It  occurs  in  both  California  and  Arizona.  The  cause  is  unknown.  It 
is  characterized  by  sudden  wilting  of  leaves  and  dying  back  of  twigs 


Fig.   14. — Gumming    (at   the   point    of   the   arrow)    on   lemon   bark,    due   to 
Diplodia  sp.  following  a  severe  frost  injury. 

or  small  branches  to  a  distance  of  12  to  24  inches  from  their  tips.  At 
the  base  of  the  dead  portion  the  bark  splits  and  gum  oozes  out  in 
considerable  quantities.  It  appears  to  occur  most  often  after  periods 
of  hot  dry  weather  and  has  been  known  in  California  for  many  years 
as  a  minor  trouble.  It  is  quite  distinct  from  citrus  blast  though  some- 
times confused  with  the  latter  disease. 


418 


UNIVERSITY    OP    CALIFORNIA EXPERIMENT   STATION 


A  somewhat  similar  gumming,  associated  with  the  dying  back  of 
twigs  and  branches  but  without  the  characteristic  splitting  of  the  bark, 
has  frequently  been  noted  in  good  sized  nursery  trees.  Although  certain 
organisms  have  been  found  in  the  lesions  none  of  them  have  reproduced 
the  disease  on  inoculation.  These  forms  of  twig  gumming  differ  from 
twig  blight  caused  by  the  cottony  rot  fungus,  Sclerotinia  lihertiana, 
in  the  absence  of  a  whitening  or  shredding  of  the  bark  and  of  the 
black  sclerotia  frequently  formed  in  the  bark  attacked  by  that  fungus. 


Fig.  15. — Cross-section  of  a  green  orange  from  a  tree  affected  with  exan- 
thema. The  darked  areas  between  the  septa  next  to  the  core  indicate  the 
presence  of  clear  gum  that  has  taken  the  place  of  the  normal  tissue. 


EXANTHEMA 

Nature  and  Symptoms. — Exanthema,  or  die-back  as  it  is  commonly 
called  in  Florida,  is  often  classed  as  a  gum  disease  although  gum  for- 
mation is  not  always  a  conspicuous  feature.  The  clear  gum  seen  in 
connection  with  exanthema  exudes  only  from  gum  pockets  on  the 
twigs,  or  is  formed  internally  near  the  center  of  the  fruit  at  the 
angles  of  the  segments  (fig.  15).  Dark  excrescences  and  multiple  buds 
on  the  branches,  dying  back  of  terminal  branches,  compact  shortened 
growth  and  dark  irregular  reddish  brown  patches  on  the  surface  of 
the  fruit  are  other  symptoms. 


Bulletin  360]     quM  DISEASES  OF  CITRUS  TREES  IN  CALIFORNIA  419 

The  disease  has  been  considered  to  be  the  result  of  a  nutritional 
disturbance,  but  the  actual  cause  is  unknown.  Localized  areas  in 
California  orchards  showing  exanthema  have  frequently  been  noted 
where  sheep  corrals  or  cattle  barns  had  been  located  in  previous  years. 
It  has  also  been  found  in  other  instances  to  correspond  to  spots  where 
the  top  soil  had  been  graded  off  in  leveling  the  orchard  before  plant- 
ing. Although  the  use  of  large  amounts  of  organic  nitrogenous  fertil- 
izers, such  as  stable  manure,  dried  blood  and  cottonseed  meal  have 
been  considered  to  be  contributing  factors  in  inducing  exanthema  on 
some  soils  in  Florida,  these  fertilizers  have  not  been  observed  to 
encourage  the  disease  under  California  conditions. 

CONTKOL 

In  many  cases  in  California  where  exanthema  has  been  trouble- 
some in  young  trees  in  localized  areas,  the  trees  have  outgrown  the 
trouble  without  any  special  treatment.  In  cases  where  the  contribut- 
ing conditions  are  such  as  may  be  economically  removed  by  the  grower 
the  remedy  is,  of  course,  obvious. 

Exanthema  is  of  such  minor  importance  in  California  that  no  study 
has  yet  been  made  of  special  methods  of  control.  The  methods  used 
in  Florida  cannot  be  recommended  for  California  without  trial  as  the 
disease  is  here  manifested  under  such  widely  different  conditions. 

OTHER  MINOR  FORMS  OF  GUMMING 

In  addition  to  the  gum  diseases  already  discussed  there  frequently 
occur  minor  forms  of  gumming,  some  of  which  have  been  shown  to  be 
induced  by  microorganisms  while  other  forms  are  brought  about  by 
insect  injuries,  chemical  stimuli,  or  in  certain  cases  by  physical  effects 
of  the  environment. 

Most  of  these  forms  of  gumming  cannot  be  classed  as  definite 
diseases  and  are  of  minor  importance  commercially  as  compared  with 
the  preceding  diseases. 

PENICILLIUM  EOSEUM 

Gumming  due  to  Penicillium  roseum.  This  fungus,  which  forms 
small  pink  tufts  of  hyphae  and  spores  on  the  surface  of  lemon  bark, 
is  capable  of  inducing  a  small  amount  of  gum  exudation  and  death  of 
small  areas  of  bark,  as  has  been  shown  by  inoculations  with  pure 
cultures.  It  has  been  found  as  a  secondary  fungus  associated  with 
Botrytis  gummosis  and  shell  bark  on  lemon  trunks,  mostly  in  the 
moister  coastal  sections  of  California. 


420  UNIVERSITY    OF    CALIFORNIA FJiPERIMENT   STATION 

FUSAEIUM 

Many  species  of  this  genus  of  fungus  produce  pinkish  to  red  pus- 
tules or  masses  of  spores  within  the  surface  layer  of  dead  bark  of 
lemon  trees.  In  general  appearance  and  color  these  red  masses  look 
to  the  unaided  eye  much  like  those  of  Penicillium  roseum.  This  fungus 
also  has  been  shown  to  be  capable  of  inducing  the  formation  of  a  small 
amount  of  gum,  and  of  causing  a  very  limited  amount  of  injury  to 
bark  tissue  adjoining  a  wound  when  inserted  into  cuts  on  sound  tissue, 
but  did  not  produce  definite  diseased  lesions.  It  has  frequently  been 
found  associated  with  brown  rot  (Pythiacystis)  gummosis  and  appears 
to  be  capable  of  increasing  the  severity  of  this  gummosis  when  associ- 
ated with  Pythiacystis  citroplitliora  (as  has  previously  been  mentioned 
under  ''brown  rot  gummosis")  but  when  acting  alone,  Fusarium 
appears  to  be  of  minor  importance  in  gummosis.  It  should  be  stated, 
however,  that  Barrett^^  has  found  species  of  Fusarium  constantly 
associated  with  the  'dry  root  rot'  of  citrus  in  connection  with  which 
considerable  gumming  is  frequently  noted.  Although  the  disease  has 
not  been  reproduced  by  inoculation,  this  fungus  is  thought  to  be  an 
important  factor  in  the  development  of  dry  root  rot  of  citrus. 

ALTEENAEIA 

A  species  of  Alternaria  similar  to  Alternaria  citri  has  very 
frequently  been  found  in  bark  tissue  associated  with  minor  forms  of 
gumming  on  lemon  trees.  Inoculation  with  pure  cultures  has  shown 
that  it  may  sometimes  induce  slight  gum  exudation  and  very  slight 
injury  to  tissue  adjacent  to  cuts  into  which  spores  were  inserted. 
Small  green  immature  navel  oranges  affected  with  black  rot  frequently 
show  gum  exudation  at  the  navel  cavity  apparently  due  to  the 
presence  of  Alternaria  citri. 

Gumming  due  to  Bacterium  citriputeale  C.  0.  Smith.  If,  during 
the  season  for  citrus  blast  and  black  pit  attack,  the  weather  becomes 
warmer  than  usual,  small  drops  of  gum  are  apt  to  form  at  the  edges 
of  the  twig  lesions  caused  by  the  citrus  blast  bacterium.  Slight  gum- 
ming has  been  noted  also  on  cuttings  inoculated  with  this  bacterium 
and  held  in  moist  chambers  over  a  free  water  surface  at  constant 
temperatures  of  80°  and  93.4°F.  (30°  and  34°C.),  but  not  at  tempera- 
tures below  this.  Control  punctures  produced  no  gum.  Large  light- 
green  lemon  fruits  also  develop  gum  by  inoculation  with  the  same 
organism  under  the  same  conditions,  but  not  in  the  uninoculated 
punctures. 

52  Barrett,  J.  T.,  Dry  root  rot.     Calif.  Citrus  Inst.,  First  Ann.  Kept.  1919- 
1920,  p.  157,  1920. 


Bulletin  360]     Gr:M  DISEASES  OF  CITRUS  TREES  IN  CALIFORNIA  421 

It  is  probable  that  various  other  fungi  are  responsible  for  minor 
forms  of  gumming  in  citrus.  Observation  has  indicated  that  under 
certain  conditions  ArmiUaria  mellea,  Schizophyllus  commune  and 
other  wood  rotting  fungi  are  minor  agents  in  bringing  about  small 
gum  exudations. 

GUMMING  ASSOCIATED  WITH  INSECT  INJUEIES 
The  injuries  to  citrus  made  by  a  number  of  insects  are  frequently 
followed  by  gumming,  usually  slight  in  amount.  Small  drops  of  gum 
may  form  on  fruit  at  injuries  produced  by  the  orange  tortrix  {Tortrix 
citrana),  and  on  small  tree  trunks  and  limbs  from  grasshopper,  katy- 
did and  other  insect  injuries.  Gumming  has  frequently  been  noted 
on  twigs  badly  infested  with  the  California  red  scale.  To  what  extent 
this  gumming  may  be  due  to  secretions  of  the  insects  or  to  the  entrance 
of  microorganisms  at  the  time  of  injury  is  uncertain.  Our  negative 
results  from  mechanical  injuries  to  citrus  kept  sterile  and  free  from 
chemical  stimuli  would  indicate  that  this  gumming  was  probably  not 
due  to  the  injury  or  wound  in  itself. 

GUMMING   ASSOCIATED    WITH    CHEMICAL    STIMULI 

Among  the  chemical  stimuli  that  have  been  seen  to  result  in  gum 
formation  occasionally  in  citrus  orchards  may  be  mentioned:  (1) 
liquid  hydrocyanic  acid  spilled  on  the  soil  near  the  roots  of  trees;  (2) 
hydrocyanic  acid  gas  used  in  fumigation;  (3)  spray  mixtures  contain- 
ing  copper  sulfate  not  properly  neutralized  with  lime  or  containing 
other  toxic  substances;  (4)  a  poison  containing  arsenic  in  contact  with 
the  bark,  etc. 

Hydrocyanic  acid  coming  in  contact  with  a  large  main  root  usually 
causes  the-  death  of  a  strip  of  limited  section  of  the  bark  on  the  trunk, 
and  even  on  the  limbs  and  branches  leading  up  from  this  portion  of 
the  trunk.  This  frequently  results  in  considerable  gumming  adjacent 
to  the  killed  strip  and  is  sometimes  mistaken  for  a  definite  gum  disease. 
Under  some  conditions  the  shock  to  citrus  trees  produced  by  fumiga- 
tion with  hydrocyanic  acid  gas  appears  to  be  responsible  for  initiating 
gumming  on  the  tree  trunks  of  orange  trees  especially.  The  pressure 
in  small  gum  pockets  formed  near  the  cambium  produces  small  rifts 
in  the  bark  through  which  the  gum  exudes  in  small  drops,  sometimes 
in  numerous  places  on  the  trunk.  Sometimes  coils  or  strings  of  gum 
will  be  pushed  out  of  small  openings  one  to  two  weeks  after  fumiga- 
tion. Such  gumming  is  usually  temporary  and  the  places  where  the 
gum  has  formed  quickly  close  up  and  fill  with  new  tissue  without 
noticeable  injury  to  the  tree. 


422  UNIVERSITY    OF    CALIFORNIA F^PERIMENT    STATION 

Copper  sulfate  solution  if  spilled  on  the  soil  over  a  root  will  act 
in  much  the  same  way  as  the  hydrocyanic  acid  in  killing  strips  of  bark 
and  resulting  in  gumming. 

Ant  poisons  containing  arsenic  when  allowed  to  leak  out  of  the 
containers  frequently  kill  patches  of  bark  and  set  up  gumming. 

PHYSICAL  EFFECTS   OF  THE  ENVIEONMENT 

Physical  effects  do  not  appear  to  be  as  a  rule  more  than  contribut- 
ing conditions  for  gumming  in  citrus  under  California  conditions, 
other  stimuli  acting  as  the  immediate  cause. 

Mechanical  Injuries. — The  writer  has  not  been  able  to  induce  gum 
exudation  on  healthy  citrus  trees  by  mechanical  injuries  alone,  pro- 
vided the  injuried  portions  are  kept  clean  and  reasonably  free  from 
contamination  with  microorganisms  or  unusual  chemical  substances. 
The  following  kinds  of  wounds  were  made :  cuts  vertically  or  hori- 
zontally through  the  bark,  augur  holes  with  and  without  glass  or 
wooden  plugs,  bruises  made  by  heavy  and  light  blows  from  a  black- 
smith's  hammer,  long  horizontal  and  vertical  slits  through  the  bark 
areas,  of  bark  cut  away,  etc.  All  such  injuries  kept  clean,  healed  in 
the  usual  way  without  gumming.  When  purposely  infected  with 
Botrytis  cinerea  or  other  injurious  fungi,  however,  gumming  resulted. 

Burning  and  Freezing. — Sunburning  and  freezing  are  not  import- 
ant factors  in  themselves  in  initiating  gum  formation  in  citrus,  al- 
though they  do  act  as  contributing  conditions  by  allowing  the  entrance 
of  organisms  such  as  Botrytis  cinerea,  Sclerotinia  lihertiana  or  other 
fungi  which,  after  becoming  established  in  the  injured  tissue,  may 
advance  rapidly  and  induce  gumming  in  tissue  apparently  sound. 
Freezing  and  sunburning  often  get  credit  for  initiating  the  gumming 
when  they  have  merely  opened  up  the  way  for  its  initiation  by  other 
agencies. 

SUMMARY 

For  the  sake  of  brevity  this  summary  deals  with  average  conditions. 
Allowance  must  therefore  be  made  for  variations  in  treatment  for 
unusual  or  abnormal  conditions.  Consult  the  discussions  in  the  text 
for  more  detailed  statements. 

Pythiacystis  (Brown  Rot)  Gummosis. — Prevented  by  pulling  the 
soil  away  from  the  base  of  the  tree  trunk  until  the  tops  of  the  first 
main  roots  are  exposed;  by  keeping  the  soil  next  to  the  trunk  from 
becoming  excessively  wet ;  by  avoiding  injuries  to  the  bark ;  by  paint- 
ing the  trunks  with  fungicide  such  as  Bordeaux  paste,  and  by  using 


Bulletin  360]     qxJM  DISEASES  OF  CITRUS  TREES  IN  CALIFORNIA  423 

sour-orange  stocks  budded  high  for  all  new  plantings,  especially  on 
heavy  clay  soils.  Treated,  when  not  too  far  gone,  by  cutting  away  the 
invaded  killed  bark,  but  not  necessarily  the  outer  gummous  zone,  and 
painting  the  wound  with  a  suitable  fungicide ;  by  scraping  away  any 
outer  layers  of  dead  bark ;  by  painting  exposed  portions  of  wood  (after 
healing  of  edges  begins)  with  benzine-asphalt  paint  or  other  suitable 
covering ;  by  cutting  back  the  tops  on  trees  severely  affected ;  by 
inarching  or  bridge  grafting  in  certain  cases. 

Botrytis  and  Sclerotinia  Gummosis. — Prevented  by  the  same 
methods  as  to  soil,  water,  injuries,  fungicides  and  use  of  sour-orange 
stocks  as  in  case  of  previous  disease.  Treated  by  cutting  out  the  dead 
bark  to  the  wood  and  by  scraping  off  only  the  outer  bark  beyond  this 
where  the  inner  layer  is  not  killed,  and  by  painting  with  fungicide 
as  in  the  previous  disease. 

Psorosis  (Scaly  Bark)  of  Orange  Trees. — Methods  of  prevention 
not  definitely  known.  Treated  by  scraping  away  the  outer  affected 
bark  of  lesions  in  the  first  and  second  stages,  scraping  lightly  the 
bark  not  j^et  visibly  affected  to  a  distance  of  6  to  8  inches  beyond  the 
lesion  in  each  direction  and  applying  Bordeaux  paste  or  other  suitable 
fungicides ;  by  cutting  out  certain  badly  affected  limbs  altogether ;  by 
eliminating  certain  of  the  worst  trees  in  the  third  stage  of  the  disease 
or  cutting  them  off  below  the  diseased  part  where  this  is  possible. 
Treatment  must  vary  much  according  to  the  stage  of  the  disease.  ^  ( See 
discussion  of  various  stages.) 

Diplodia  Gumming  and  Twig  Gumming. — Treated  by  eliminating 
the  parts  affected  and  by  treating  cuts  with  fungicide  followed  by 
paint  where  wounds  are  large. 

Exanthema. — Little  is  known  as  to  its  real  cause,  and  no  one  defin- 
ite method  of  prevention  or  control  can  be  suggested  for  California 
conditions  except  to  eliminate  certain  supposed  contributing  condi- 
tions where  this  is  possible.     (See  previous  discussion.) 

Minor  Forms  of  Gumming. — Not  many  of  these  are  sufficiently 
important  to  require  special  attention  either  as  to  prevention  of  treat- 
ment. Where  they  are  due  to  organisms  most  of  the  same  principles 
apply  as  are  given  for  the  previous  diseases.  Some  forms  are  depend- 
ent upon  conditions  that  cannot  be  controlled  but  recovery  often 
follows  a  change  in  the  contributing  conditions. 


STATION  PUBLICATIONS  AVAILABLE  FOR  FREE  DISTRIBUTION 


BULLETINS 


No. 

253.  Irrigation  and  Soil  Conditions  in  the 
Sierra  Nevada  Foothills,  California. 

261.  Melaxuma    of    the    Walnut,     "Juglans 

regia." 

262.  Citrus   Diseases   of   Florida   and   Cuba 

Compared  with  these  of  California. 

263.  Size  Grades  for  Ripe  Olives. 

268.   Growing  and  Grafting  Olive  Seedlings. 

270.  A  Comparison  of  Annual  Cropping,  Bi- 
ennial Cropping,  and  Green  Manures 
on  the  Yield  of  Wheat. 

273.  Preliminary  Report  on  Kearney  Vine- 
yard Experimental  Drain. 

275.  The  Cultivation  of  Belladonna  in  Cali- 

f-ornia. 

276.  The   Pomegranate. 

278.  Grain   Sorghums. 

279.  Irrigation  of  Rice  in  California. 

280.  Irrigation  of  Alfalfa  in  the  Sacramento 

Valley. 
283.  The  Olive  Insects  of  California. 

285.  The  Milk  Goat  in  California. 

286.  Commercial    Fertilizers. 

287.  Vinegar  from  Waste  Fruits. 
294.   Bean    Culture   in    California. 

297.  The  Almond  in   California. 

298.  Seedless  Raisin  Grapes. 

299.  The  Use  of  Lumber  on  California  Farms. 
304.   A  study  on  the  Effects  of  Freezes  on 

Citrus   in   California. 
308.   I.  Fumigation  with  Liquid  Hydrocyanic 
Acid.  II.  Physical  and  Chemical  Prop- 
erties of  Liquid  Hydrocyanic  Acid. 

312.  Mariout  Barley. 

313.  Pruning  Young   Deciduous  Fruit  Trees. 

316.  The   Kaki   or   Oriental   Persimmon. 

317.  Selections   of   Stocks   in   Citrus    Propa- 

gation. 
319.   Caprifigs   and  Caprification. 
321.   Commercial  Production  of  Grape  Syrup. 

324.  Storage  of  Perishable  Fruit  at  Freezing 

Temperatures. 

325.  Rice  Irrigation  Measurements  and  Ex- 

periments     in      Sacramento      Valley, 
1914-1919. 


No. 

328. 
331. 
332. 
334. 

335. 

336. 

337. 
339. 

340. 

341. 
342. 
343. 
344. 

345. 

346. 
347. 

348. 
349. 

350. 
351. 
352. 

353. 
354. 
355. 
356. 

357. 


358. 

359. 
360. 


Prune  Growing  in  California. 

Phylloxera-Resistant  Stocks. 

Walnut  Culture  in  California. 

Preliminary  Volume  Tables  for  Second- 
Growth  Redwoods. 

Cocoanut  Meal  as  a  Feed  for  Dairy 
Cows  and  Other  Livestock. 

The  Preparation  of  Nicotine  Dust  as 
an  Insecticide. 

Some  Factors  of  Dehydrater  Efficiency. 

The  Relative  Cost  of  Making  Logs  from 
Small    and   Large   Timber. 

Control  of  the  Pocket  Gopher  in  Cali- 
fornia. 

Studies  on  Irrigation  of  Citrus  Groves. 

Hog  Feeding  Experiments. 

Cheese  Pests  and  Their  Control. 

Cold  Storage  as  an  Aid  to  the  Market- 
ing of  Plums. 

Fertilizer  Experiments  with  Citrus 
Trees. 

Almond    Pollination. 

The  Control  of  Red  Spiders  in  Decidu- 
ous Orchards. 

Pruning  Young  Olive  Trees. 

A  Study  of  Sidedraft  and  Tractor 
Hitches. 

Agriculture  in  Cut-over  Redwood  Lands. 

California  State  Dairy  Cow  Competition. 

Further  Experiments  in  Plum  Pollina 
tion. 

Bovine  Infectious  Abortion, 

Results  of  Rice  Experiments  in   1922. 

The  Peach  Twig  Borer. 

Observations  on  Some  Rice  Weeds  in 
California. 

A  Self-mixing  Dusting  Machine  for 
Applying  Dry  Insecticides  and 
Fungicides. 

Black  Measles,  Water  Berries,  and 
Related    Vine    Troubles. 

Fruit  Beverage  Investigations. 

Glim  Diseases  of  Citrus  Trees  in  Cali- 
fornia. 


CIRCULARS 

No.  No. 

70.   Observations    on    the    Status    of    Corn  166. 

Growing  in  California.  167. 

82.  The  Common  Ground  Squirrel  of  Call-  169. 

fornia.  170. 
87.  Alfalfa. 

110.  Green  Manuring  in  California.  172. 

111.  The  Use  of  Lime  and  Gypsum  on  Cali-  173. 

fornia  Soils. 

113.   Correspondence  Courses  in  Agriculture.  174. 

117.  The    Selection    and    Cost    of    a    Small  175. 

Pumping  Plant. 

127.   House  Fumigation.  178. 

136.  Melilotus    indica    as    a    Green-Manure  179. 

Crop  for  California. 

144.   Oidium  or  Powdery  Mildew  of  the  Vine.  182. 
148.   "Lungworms." 

151.  Feeding  and  Management  of  Hogs.  183. 

152.  Some  Observations  on  the  Bulk  Hand-  184. 

ling  of  Grain  in   California.  188. 

155.   Bovine  Tuberculosis.  190. 

157.   Control  of  the  Pear  Scab.  193. 

159.  Agriculture  in   the  Imperial   Valley.  198. 

160.  Lettuce  Growing  in  California.  199. 

161.  Potatoes  in   California.  201, 
165,  Fundamentals   of   Sugar   Beet  Culture  202, 

under  California  Conditions. 


The   Country  Farm   Bureau. 
Feeding  Stuffs  of  Minor  Importance. 
The   1918   Grain  Crop. 
Fertilizing  California  Soils  for  the  1918 

Crop. 
Wheat  Culture. 
The    Construction    of    the    Wood-Hoop 

Silo. 
Farm   Drainage  Methods. 
Progress  Report  on  the  Marketing  and 

Distribution  of  Milk, 
The  Packing  of  Apples  in  California. 
Factors    of    Importance    in    Producing 

Milk  of  Low  Bacterial  Count. 
Extending  the  Area  of  Irrigated  Wheat 

in   California  for  1918. 
Infectious  Abortion  in  Cows. 
A  Flock  of  Sheep  on  the  Farm. 
Lambing   Sheds. 

Agriculture   Clubs   in   California. 
A  Study  of  Farm  Labor  in  California. 
Syrup  from   Sweet  Sorghum, 
Onion  Growing  in  California, 
Helpful  Hints  to  Hog  Raisers. 
County    Organizations    for   Rural    Fire 

Control. 


CIRCULARS— Continued 


No. 

203.   Peat  as  a  Manure  Substitute.   . 

205.  Blackleg. 

206.  Jack  Cheese. 

208.  Summary  of  the  Annual  Reports  of  the 

Farm  Advisors  of  California. 

209.  The  Function  of  the  Farm  Bureau. 

210.  Suggestions  to  the  Settler  in  California. 
212.   Salvaging  Rain-Damaged  Prunes. 

214.  Seed  Treatment  for  the  Prevention  of 

Cereal   Smuts. 

215.  Feeding  Dairy  Cows  in  California. 

217.  Methods    for    Marketing   Vegetables   in 

California. 

218.  Advanced    Registry    Testing    of    Dairy 

Cows. 

219.  The  Present   Status  of  Alkali. 

224,  Control    of    the    Brown    Apricot    Scale 

and  the  Italian  Pear  Scale  on  Decid- 
uous Fruit  Trees. 

225.  Propagation  of  Vines. 

228.  Vineyard   Irrigation  in   Arid  Climates. 
230.   Testing  Milk,    Cream,    and    Skim   Milk 
for  Butterfat. 

232.  Harvesting    and    Handling    California 

Cherries  for  Eastern  Shipment. 

233.  Artificial  Incubation. 

234.  Winter  Injurv  to  Young  Walnut  Trees 

during  1921-22. 

235.  Soil  Analysis  and  Soil  and  Plant  Inter- 

relations. 

236.  The  Common  Hawks  and  Owls  of  Cali- 

fornia  from    the     Standpoint  of  the 
Rancher. 


No, 

237.  Directions  for  the  Tanning  and  Dress- 

ing of  Furs. 

238.  The  Apricot  in  California. 

239.  Harvesting  and  Handling  Apricots  and 

Plums  for  Eastern   Shipment. 

240.  Harvesting    and    Handling    Pears    for 

Eastern    Shipment. 

241.  Harvesting  and  Handling  Peaches  for 

Eastern    Shipment. 

242.  Poultry  Feeding. 

244.  Central  Wire  Bracing  for  Fruit  Trees. 

245.  Vine  Pruning  Systems. 

246.  Desirable   Qualities   of  California   Bar- 

ley for  Export. 

247.  Colonization   and  Rural  Development. 

248.  Some  Common  Errors  in  Vine  Pruning 

and  Their  Remedies. 

249.  Replacing  Missing  Vines. 

250.  Measurement    of    Irrigation    Water    on 

the   Farm. 

251.  Recommendations  Concerning  the  Com- 

mon     Diseases      and      Parasites      of 
Poultry    in    California. 

252.  Supports  for  Vines. 

253.  Vineyard   Plans. 

254.  The  Use  of  Artificial  Light  to  Increase 

Winter  Egg  Production. 

255.  Leguminous   Plants   as  Organic  Fertil- 

izer in  California  Agriculture. 

256.  The  Control  of  Wild  Morning  Glory, 

257.  The  Small-Seeded  Horse  Bean. 

258.  Thinning  Deciduous  Fruits. 

259.  Pear  By-products. 


